Metal nanowire based bandpass filter arrays in the optical frequency range

ABSTRACT

A metal nanowire array acts as a band pass filter in the visible to IR range. The filter may be used in a monochromator, spectrometer, color camera, analyte detector or other devices.

This application is a continuation-in-part application based onPCT/US2004/023499, filed Jul. 22, 2004, which claims priority from U.S.Provisional Application No. 60/492,954, filed Aug. 6, 2003, U.S.Provisional Application No. 60/492,956, filed Aug. 6, 2003, and U.S.Provisional Application No. 60/492,955, filed Aug. 6, 2003. Thisapplication also claims the benefit of priority from U.S. ProvisionalApplication No. 60/655,430 filed Feb. 24, 2005. Each of the foregoingapplications is incorporated herein by reference in its entirety.

The U.S. government may have certain rights in this invention pursuantto grant number 00014-99-0663 from the Office of Naval Research.

FIELD OF THE INVENTION

The present invention is directed generally to optical devices and moreparticularly to nanostructured optical devices and methods of making thedevices.

BACKGROUND OF THE INVENTION

The electromagnetic interactions with metal wires have been a subject ofresearch over hundred years. In 1888, for example, Heinrich Hertz firstused a metal wire grid as a polarizer to test the properties of thenewly discovered radio wave. For example, see H. Hertz, ELECTRIC WAVES(Macmillan and Company, London, 1893) at page 177. Since then, the workin the radio frequency regime has been extended to a broad spectralrange down to the optical regime. Metal wire arrays show strongpolarization-dependent transmission and this has been utilized forpolarization filtering. For instance, see G. R. Bird and M. Parrish, J.Appl. Phys. 50, 886 (1960). Another common feature of the conventionalmetal wire arrays is that they reveal high transmission in the longerwavelength regime (i.e., long pass in terms of filter characteristics).

SUMMARY OF THE INVENTION

One embodiment of the invention relates to a band pass filter comprisinga metal nanowire array.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic top view of a device according to the firstpreferred embodiment of the present invention.

FIG. 1B is a side cross sectional view across line A-A′ in FIG. 1A.

FIG. 2A is a scanning electron microscope (SEM) image of an exemplarydevice of the present invention. The minor mesas observed on some trenchbottoms are irregularly-etched quartz surface profiles.

FIGS. 2B and 2D are graphs of the transmission spectra of devicesaccording to examples 1, 2 and 3 of the present invention.

FIG. 2C is a graph of the transmission/reflectance spectra, of devicesaccording to examples 1 and 2 of the present invention.

FIGS. 3A, 3B and 3C are FDTD simulated transmission spectra of lightthrough silver nanowire arrays (nanoslit arrays). The dimensions of thestructure are shown in the insets with the silver thickness (h), slitspacing (d) and slit width (a).

FIG. 4 is a schematic side cross sectional view of a device according tothe preferred embodiments of the present invention.

FIGS. 5 and 6 are schematic top views of devices according to thepreferred embodiments of the present invention.

FIG. 7 is a schematic side cross sectional view of a device according tothe preferred embodiments of the present invention.

FIG. 8 is a schematic side cross sectional view of an apparatus used tomake the device of FIG. 7.

FIGS. 9A and 9B are schematic side cross sectional views of a method ofmaking a device according to the preferred embodiments of the presentinvention.

FIG. 9C is a schematic top view of a holographic lithography system.

FIGS. 9D-9I are schematic three dimensional views of a method of makinga device according to the preferred embodiments of the presentinvention.

FIGS. 10A, 10B and 10C are micrographs of a method of making a nanoporearray according to the preferred embodiments of the present invention.

FIG. 10D is a schematic side cross sectional view of a device accordingto the preferred embodiments of the present invention.

FIG. 10E is a schematic side cross sectional view of an electroplatingbath used to make the device of FIG. 10D.

FIGS. 11A, 11B, 11C and 11D are schematic side cross sectional views ofa method of making a device according to the preferred embodiments ofthe present invention.

FIGS. 12A and 12B are schematic side cross sectional views of a methodof making a device according to the preferred embodiments of the presentinvention.

FIGS. 13A, 13B, and 13C are schematic side cross sectional views of amethod of making a device according to the preferred embodiments of thepresent invention.

FIGS. 15 and 18 are plots of transmission spectra through devices of thefirst preferred embodiment of the present invention.

FIG. 14 is a schematic side cross sectional view of a device accordingto the preferred embodiments of the present invention.

FIGS. 16A, 16B and 17 are micrographs of devices according to the firstembodiment of the present invention.

FIGS. 19 and 21 are side cross sectional views of devices according tothe preferred embodiments of the present invention.

FIGS. 20A and 20B are top views of devices according to the preferredembodiments of the present invention.

FIG. 22A is a perspective view and FIGS. 22B and 22C are top views of amultispectral imaging system of the preferred embodiments of the presentinvention.

FIGS. 23A and 23B are perspective views of an optical analyte detectionsystem of the preferred embodiments of the present invention.

FIG. 24 is a schematic illustration of a method of using the opticalanalyte detection system of the preferred embodiments of the presentinvention.

FIG. 25 is a schematic top view of an experimental set up for examples5-12.

FIG. 26 is a plot of transmission spectra for examples 5, 6 and 7.

FIG. 27 is a plot of transmission spectra for three prior art filters.

FIGS. 28, 29, and 30 are plots of transmission versus location on thedetector for examples 8, 9, and 10 respectively.

FIGS. 31A and 32A are schematic top views of wavelength separationdevices according to embodiments of the present invention.

FIG. 31B is a micrograph of the device of FIG. 31A.

FIG. 32B is a schematic plot of grating period versus location on thedetector for the device of FIG. 32A.

FIG. 33A is a schematic illustration of the device of example 11.

FIG. 33B is a schematic plot of grating period versus location on thedetector for the device of FIG. 33A.

FIGS. 34A, 34B and 34C are plots of transmission spectra for the deviceof example 11.

FIG. 35A is a schematic plot of grating period versus location on thedetector for the device of example 12. FIG. 35B is a plot oftransmission spectra for the device of example 12.

FIGS. 36A and 36B are plots of transmission spectra for the device ofexamples 13 and 14. FIG. 36C is a plot of transmission power versuslocation the detector for the device of example 15.

FIGS. 37A-F are FDTD simulations of the field and energy flowdistributions calculated at three different wavelengths, 540 nm (FIGS.37A and 37B), 680 nm (FIGS. 37C and 37D) and 1500 nm (FIGS. 37E and37F). A silver nanoslit array with a rectangular cross-section isassumed with 200-nm metal thickness, 370-nm period and 80-nm slit width.A TM-polarized planar wave is incident from the air side (bottom) andexits to the quartz side (top).

FIGS. 38A, 38B and 38C are FDTD simulations of time evolution of the SPpolarization charge distributions calculated at three different regimes,at 540 nm (FIG. 38A), 680 nm (FIG. 38B) and 1500 nm (FIG. 38C)wavelength. Four snap-shot images of charge distributions are displayedwith a π/2 phase interval. The lighter and darker colors along theperiphery of each metal island represent the positive and negativepolarity of charges, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present inventors discovered that a metallic nanowire arraystructure offers a well-defined narrow bandpass filter characteristic ina broad spectral range covering the visible and the near infra red(“IR”) regime. The structure utilizes the plasmonic phenomena occurringin a specially designed metal wire array in order to control and shapethe spectral profiles of optical transmission. Near-field coupling ofsurface plasmon (SP) waves in between neighboring metal wires is afactor in suppressing optical transmission in the long wavelength regimewhile allowing maximum transmission in the main passband. See, forexample, Z. Sun, Y. S. Jung, and H. K. Kim, Appl. Phys. Lett. 83, 3021(2003); Z. Sun, Y. S. Jung, and H. K. Kim, Appl. Phys. Lett. 86, 023111(2005); and international application PCT/US04/23499.

The present inventors discovered that a narrow window exists for theinterwire gap that allows for bandpass filtering. Nanowire arrays withinterwire gaps having a width of 100 nm or less, such as 30 to 100 nm,for example 50-100 nm, produce a clear bandpass filter characteristicwith bandwidth of ˜λ/10, where λ is the center wavelength of passband,in the visible to near IR range. The nanowire filters are highlyscalable in terms of physical dimension (down to micron size per filter)and in channel capacity (about 100 channels over a millimeter sizedspan). Densely-spaced nanowire filter arrays that show variable passbandwavelength in the array direction are provided. Chip-scale opticalspectrum analyzers may be made utilizing the nanowire array structure.

It is believed that localized surface plasmon (SP) resonance can occurat metal islands in an array of slit shaped transparent regions alignedin one direction between the metal islands, but not in an array ofnon-slit shaped apertures in a metal film. The transmission of radiationis higher through the array of slit shaped transparent regions betweenthe metal islands than through the array of non-slit shaped apertures ina metal film.

As shown in FIGS. 1A and 1B, the nanowire array 1 comprises a pluralityof slit shaped radiation transparent regions 7 located between theradiation opaque metal nanowires 5. The nanowires 5 may comprise metalislands located over a radiation transparent substrate 3. FIG. 1Billustrates a side cross sectional view of the device 1 along line A-A′.

In a preferred aspect of the present invention, the metal islands 5 areformed using lithography and/or self assembly to simplify processing andto increase the precision of the optical device. In one preferred aspectof the present invention, the metal islands 5 spaced apart by radiationtransparent regions 7 are formed by self assembly. In other words,rather than forming a metal film and patterning the film into metalislands, the spaced apart metal islands 5 are formed simultaneously orsequentially without first being part of an unpatterned metal film. Inanother preferred aspect, the metal islands comprise islands that areformed by patterning a metal film into the islands. Preferably, theislands are patterned using a lithographic method. Thus, metal islandsmade by self assembly or by photolithgraphy may comprise discrete metalislands that are not connected to each other (i.e., the metal islandsare not in direct contact with each other) or metal islands that areconnected to each other at a peripheral region of the device.

FIG. 2A shows a scanning electron microscope (SEM) image of an exemplarynanowire array containing slit shaped transparent regions (openings inFIG. 2A) between elongated nanowire islands. The nanowire array was madeby angle-depositing a silver layer on top of a mesa-etched quartzsubstrate, as will be described in more detail with respect to FIGS. 8and 9A-9I below. The Ag nanowire array of FIG. 2A shows a clear openingin the gap region.

The slit width (i.e., interwire gap) was varied in the range of 30-100nm by controlling the Ag layer thickness during deposition. FIG. 2Bshows the transmission spectra of nanowire array examples with Ag layerthickness of 120 (example 1), 200 nm (example 2) and 180 nm (example 3).

The devices of examples 1, 2 and 3 include metal islands with a gratingperiod of 370 nm. Optical transmission through the devices is measuredat a spectral range of 350-1750 nm. A beam from a multimode fiber (corediameter of 62.5 μm and a numerical aperture of 0.20) connected to anunpolarized white light source is normally incident to the metal islandarray from the substrate side. The zero-order transmission through thearray is collected with another multimode fiber placed close to the Aglayer surface (with 3-5 μm gap), and is then characterized with anoptical spectrum analyzer. The transmission measurement is repeated witha dummy sample that has the same mesa-etched quartz structure butwithout an Ag layer. The transmission through the array is thencalculated by dividing the spectrum obtained from a real sample by theone from the dummy, a process designed to void (or alleviate) theeffects of involving a mesa-etched quartz substrate structure and anoptical fiber on the measured transmission spectra.

Well-defined dips and peaks are observed in the spectral range of 350 to1750 nm. The transmission minima at around 430 or 580 nm is believed tocorrespond to the SP resonances along the planes that comprise eitherair- or substrate-side surfaces of metal islands, respectively. Thetransmission dip at around 800 nm is believed to be due to the SPresonance localized at each metal island, i.e., the resonance along theperiphery of metal island surface.

The spectra of the 120-nm-thick sample of example 1 shows hightransmission (nearly the same level as the peak transmittance of themain passband at around 650 nm) in the long wavelength range (beyond 950nm).

The spectra of the 180-nm-thick sample of example 3 shows thelong-wavelength transmission significantly decreased (from 50% level to20% transmission for TM polarization) while the main passband wasslightly higher than that of example 1 and the dip at shorter wavelengthregion remains at a nearly same level as that of example 1.

For spectra of the 200 nm thick sample of example 2, both the mainpassband and long wavelength transmission decreased compared to example3.

As shown in FIG. 2B, peak transmission of approximately 30, 15 and 35%,respectively, is observed from the 120 nm, 200 nm and 180 nm thick metalisland nanowire arrays, respectively. Considering that the incident beamis unpolarized and the TE polarization component does not transmitthrough the array having slit shaped transparent region, the maximumtransmission for TM polarization is estimated to be at least 60% orhigher. This corresponds to about a 500% transmission efficiency, whichis defined as the optical power transmitted through a slit divided bythe incident power impinging upon the slit area.

As shown in FIG. 2B, the main peak (i.e., the peak which corresponds tothe main passband wavelength range) shifts from 660 to 690 nm as the Agisland thickness is increased from 120 to 200 nm. The peak width alsonoticeably increased with the increased Ag thickness. Thesecharacteristics of the transmission spectra, i.e., the main peak'sred-shift and the peak width increase, is the opposite of thecharacteristics of the transmission spectra in an 2D array of aperturesin a metal film, in which the main peak initially blue-shifts withreduced peak-width and then the peak position and width remain constantas the metal thickness is further increased.

The optical transmission through nanowire arrays is analyzed byemploying a FDTD method. FIG. 37A shows the field (the H_(z) component,parallel to the grating lines) distribution around a nanowire arrayanalyzed at 540 nm wavelength, which corresponds to the SP resonancepoint at metal/dielectric interface. In this simulation, the followingdimensions were assumed for the nanowire structure: grating period of370 nm, slit width of 80 nm, and Ag layer thickness of 200 nm. Arectangular cross-section of metal islands formed on a quartz substratewith constant gap was assumed for simplicity of analysis. The dielectricconstants of Ag and quartz used in this simulation are from E. D. Paliked., HANDBOOK OF OPTICAL CONSTANTS OF SOLIDS (Academic, New York, 1998).A transverse-magnetic (TM)-polarized plane wave from the air side isassumed to be incident to the bottom surface of the Ag slit array inthis image. Fields of significant strength appear at the exit side(quartz side) of metal with their distribution highly confined andlocalized around the slits. The fields, however, do not propagate beyondthe near-field region, and therefore it results in null transmission oflight in the far field regime. FIG. 37B shows the energy flow around theslit structure. A close-up image is shown in this Figure in order toreveal details in the near-field region around a slit. A significantamount of energy flows out of the slits, but it sharply deflects towardsthe metal surface upon exiting the slits. The exited energy flows backand forth along the in-plane direction on the metal surface. The spatialperiod of the energy flow distribution matches the field distributionshown in FIG. 37A, equal to the array period. An energy flow componentnormal to the substrate exists in some near field region, but it doesnot propagate away into the far field region.

FIGS. 37C and 37D show the field and energy flow distributions,respectively, analyzed at 680 nm wavelength, which corresponds to thepeak transmission point in the measured spectrum shown in FIG. 2B. Thefield distribution confirms that the nanowire array is highlytransmissive for the TM-polarized incident wave at this wavelength. Theenergy flow distribution reveals that the incident power is wellfunneled into each slit. The effective area of funneling is found to beextensive covering nearly entire spacing between slits. The transmittedpower radiates away from slits and forms a propagating wave in the farfield region. The fields localized on the slit array surface quicklyevolve into planar wavefronts in less than one wavelength ofpropagation.

At 1500 nm wavelength (FIGS. 37E and 37F), optical transmission is foundsignificantly decreased. A fraction of incident power transmits througheach gap, but the funneling effect is significantly reduced. Thissuggests that near-field coupling between neighboring metal islands mayoccur altering the SP dynamics and the far-field distribution of opticalfields. At narrow slit width, the surface plasmon fields localized ateach metal island are expected to couple each other via a tunnelingprocess. See M. Quinten, A. Leitner, J. R. Krenn, and F. R. Aussenegg,Opt. Lett. 2331 (1998); J. P. Kottmann and O. J. F. Martin, Opt. Lett.26, 1096 (2001); S. A. Maier, M. L. Brongersma, P. G. Kik, and H. A.Atwater, Phys. Rev. B 65, 193408 (2002).

Once strong coupling occurs between metal islands, the islands becomevirtually connected (despite a gap) from the surface plasmon oscillationpoint of view. SP propagation along the slits will eventually be blockedand the transmission spectra would show profiles similar to those ofmetal without slits.

Surface plasmon is a collective oscillation of electrons. The spatialdistribution of SP polarization charges and their time evolution can becalculated from the field distribution information in conjunction withthe following relationship, ∇·(∈E)=ρ, where ∈ is the dielectricconstant, E is the electric field, and ρ is the charge density. Since nofree charges are assumed (ρ=0), the SP polarization charge density, −∇·Pis equal to ∈_(o)∇·E, where P is the SP polarization.

FIGS. 38A-C show the FDTD simulation of images of the polarizationcharge distributions analyzed at different regimes of SP resonance. Thetime evolution characteristic is illustrated with four snap-shot imagesof the charge distributions calculated with a time interval of T/4,where T is the period of oscillation of light. The images clearly revealthat the polarization charges are highly localized along the metalsurfaces, consistent with the nature of SP waves.

At 540 nm wavelength in FIG. 38A, the polarization charge distributionreveals an interesting feature in its time evolution characteristic. Thepolarization charges along the metal/substrate (quartz in thissimulation) interface show a stable in-plane oscillation across aspatial period that matches the slit spacing. This indicates that the SPwaves propagating along the metal/substrate surface form a standing waveoscillation and does not allow for far-field transmission of theincident power. This analysis result supports the proposition that theSP resonance at the dielectric (quartz) side of metal surface isresponsible for the transmission dip observed at this wavelength.

At 680 nm wavelength shown in FIG. 38B, the polarization charges show atendency towards a quadrupolar distribution along the periphery of eachmetal island. SP propagation along metal surface is determined byvarious factors, such as the size and shape of metal islands. See C. F.Bohren and D. R. Huffman, ABSORPTION AND SCATTERING OF LIGHT BY SMALLPARTICLES (Wiley, New York, 1983). In the case of FIG. 38B, the size ofmetal islands is in the Mie scattering regime, so that the retardationeffect cannot be neglected. Having a highly anisotropic shape with sharpcorners, the metal islands are expected to show a complex behavior of SPoscillations, such as resonances occurring at the sections of peripherydivided by corner points. See J. P. Kottmann, O. J. F. Martin, D. R.Smith, and S. Schultz, Phys. Rev. B 64, 235402 (2001)). Despite thecomplexity, a quadrupole nature of polarization charge distribution isclearly observed along with a traveling wave characteristic of SPoscillations in this simulation.

At 1500 nm wavelength of FIG. 38C, the polarization charges show quitedifferent distributions. This figure shows a dipolar distribution alongthe array direction with polarization charges mostly localized at theslit walls. In the case of relatively wide slits, individual metalislands behave like a dielectric sphere of certain polarizability, andthus the incident wave transmits without much reflection or attenuation.In the case of narrow slits, significant coupling occurs betweenneighboring islands and therefore the metal islands respond as a groupor a chain. This causes significant blocking of light since the nanowirearray asymptotically approaches the case of a continuous metal layerwithout any slit. The optical interaction in the nanowire array(localization of SPs at metal islands and their coupling, and thetransmission characteristic) is reminiscent of metallic particle arraysand aggregates, whose transmission characteristic is also dependent ofparticle size, shape and spacing. See U. Kreibig and M. Vollmer, OPTICALPROPERTIES OF METAL CLUSTERS (Springer-Verlag, Berlin, 1994)). Couplingbetween the surface plasmons in neighboring islands is expected todepend on the degree of overlap of the SP fields across a slit, which isdetermined by the spatial extension of SP fields relative to gap size(i.e., slit width). See H. Raether, SURFACE PLASMONS (ed. Hohler, G.)(Springer, Berlin, 1988).

In order to substantiate the relationship between slit-width and opticaltransmission, the present inventors carried out FDTD analysis on themetal wire array structure with various different dimensions andcalculated the optical transmission. Maxwell equations were solvednumerically for the transmission problem. The problem region wasuniformly discretized with 551×551 mesh points; each mesh unitrepresents for a real dimension of 10 nm. The structure simulated has athickness of 200 nm, slit width of 80 nm and slit spacing (center tocenter) of 370 nm involves adjacent media of silica (∈=2.16) on the topside and air (∈=1) on the bottom side. The imaginary part of thedielectric constants of silver was ignored in this simulation forconvenience. The field components (Hz, Ex, Ey) were calculated in a widerange of wavelength from 400˜1500 nm. The excitation source was TMpolarized sinusoidal monochromatic wave at individual wavelengths.

While the slit spacing (center to center) was fixed to 370 nm, thesilver thickness was varied to 80,120 or 200 nm, and slit width to 50,80, or 120 nm. The number of time steps is 2000 generally unless beingspecified.

FIGS. 3A-C show the transmission spectra obtained from the simulationwith different structural conditions. FIG. 3A shows the simulation forsilver thicknesses (“h”) of 80, 120 and 200 nm with the slit spacing(“d”) and slit width (“a”) held constant at 370 nm and 50 nm,respectively. FIG. 3B shows the simulation for silver thicknesses of 80,120 and 200 nm with the slit spacing and slit width held constant at 370nm and 80 nm, respectively. FIG. 3C shows the simulation for silverthicknesses of 80, 120 and 200 nm with the slit spacing and slit widthheld constant at 370 nm and 120 nm, respectively. It should be notedthat the small fluctuations of transmittance should not be considered inthe analysis, which is due to the cavity effects introduced by non-idealabsorbing boundary conditions of the problem region in simulation.

The simulation result shows that the transmittance at longer wavelengthsclearly decreased with reduced slit width. Generally, it is also seenthat the wavelength filtering is more prominent with increased metalfilm thickness, and there is almost no clear passband for the thicknessof 80 nm (i.e., the main passband transmittance is higher for increasedfilm thickness, while transmittance at side lobes is lower withincreased film thickness). As shown in FIGS. 3A-C, the reduced slitwidth does not apparently significantly reduce the transmittance at mainpassband. At larger film thicknesses, such as 120 nm and 200 nm, themain transmission peak shifts towards longer wavelength with thickerfilm, which shows the same trend as those experimental results shown inFIG. 2B. The transmission spectra for thick films show a prominent dipat the wavelength of 540 nm, which is at the in-plane surface plasmonresonance condition at silver/silica interface.

The absence of the localized-SP-resonance-related dip at around 800-850nm in the simulation result is attributed to the following reason. Inthis simulation, it was assumed that a rectangular cross-section ofmetal wires and the sharp corners are expected to cause strongdiffraction/scattering of SP waves, which attenuates localized SPpropagation along the metal island boundary.

Thus, it is believed that localized surface plasmon (SP) resonance canoccur at metal islands in a 1D array of slit shaped transparent regionsbetween the metal islands, but not in two dimensional (2D) array ofapertures in a metal film. This localized SP resonance and propagatingmodes are thus unexpectedly present in the 1D array of slit shapedtransparent regions between the metal islands, but not in 2D array ofapertures in a metal film. Furthermore, the transmission of radiation ishigher through the 1D array of slit shaped transparent regions betweenthe metal islands than through the 2D array of apertures in a metalfilm.

Furthermore, the present inventors have discovered that in the 1D arrayof slit shaped transparent regions between the metal islands, the widthof the slit shaped transparent regions, and the metal island height(i.e., thickness) determine the transmission characteristics of these 1Darrays. Without wishing to be bound by a particular theory, the presentinventors believe that the localized SP resonance is responsible forthis effect. When the transparent region width is within a preferredrange, high transmission in the main passband wavelength and lowtransmission in the long wavelength range may be achieved. The preferredrange is between about one and about three times the penetration depthof SP fields in the metal islands. Preferably, the width of thetransparent regions is 100 nm or less, such as greater than 30 nm andless than 100 nm. This range is preferable for the visible spectrum oflight, and the preferred slit width will proportionally increase for thelonger wavelength regime. This feature of 1D metal island/slit shapedtransparent region arrays may be used in designing and/or developingspectral characteristics of filters, such as to form optical pass-bandfilters with a narrow passband width. In contrast, when the transparentregion width is greater than this preferred range, the transmission at amain passband wavelength and in the long wavelength range is high, asshown in FIG. 3C. When the transparent region width is less than 30 nm,it results in low transmission at the main passband wavelength and inthe long wavelength range.

Without wishing to be bound by a particular theory, the presentinventors believe that two types of surface plasmon excitation areresponsible for the characteristics of the radiation transmitted throughthe slit shaped transparent regions between metal islands or in themetal film: 1) the SP resonance along the planes that comprise eitherthe metal/air or metal/substrate interfaces, and 2) the SP resonancelocalized along the surface that encloses each metal island separated bythe slit shaped transparent region (i.e., the metal island sidewalls ortransparent region sidewalls in a metal film).

The present inventors also believe that a peak transmission occurs in adevice where the localized SP resonance is slightly off-tuned from theplasmon resonance at the metal/substrate surface. It is then expectedthat in such devices, the main passband transmission will remain highwhile the long-wavelength transmission will be low. Furthermore, thethickness (i.e., height) of the metal islands affects the width of themain passband peak. Generally, the width of the main passband peakdecreases with a decreasing metal island thickness. It should be notedthat the device ideally contains one passband at one peak wavelength.However, the devices may contain more than one passband with more thanone peak wavelength.

Any suitable materials may be used for the substrate 3 and the metalislands 5 of the nanowire array 1 shown in FIGS. 1A, 1B and 2A. Forexample, any radiation transparent material (i.e., visible light, UV andIR transparent material) may be used as the substrate material. Forexample, the substrate 3 may comprise glass, quartz, ceramic, plastic orsemiconductor material. The substrate 3 may comprise a plurality offilms or layers or it may comprise a unitary body.

Any metal which exhibits surface plasmon resonance effects (i.e., anegative epsilon material) may be used as the metal island 5 material.For example, metals such as silver, gold, copper and aluminum and alloysthereof which exhibit a bulk plasmon frequency about 9-10 eV, arepreferred as the metal island material.

As shown in FIG. 1A, adjacent metal islands 5 are separated by adistance 9 which is less than at least one first predeterminedwavelength of incident radiation to be provided onto the device 1.Preferably the distance 9 is less than 100 nm, such as 30 to 80 nm, forexample 40 to 60 nm. This range is preferable for the visible spectrumof light, and the preferred distance will proportionally increase forthe longer wavelength regime. The metal islands 5 are configured suchthat the incident radiation is resonant with a surface plasmon mode onthe metal islands, thereby enhancing transmission of radiation betweenthe plurality of metal islands. Preferably the transmitted radiation hasat least one peak wavelength whose transmission is enhanced by thesurface plasmon resonance.

Any suitable radiation may be used as incident radiation. For example,the incident radiation may comprise visible light, UV or IR radiation.The incident radiation may comprise radiation with a narrow wavelengthdistribution, such as radiation with a peak wavelength and narrow bandwidth around the peak wavelength, or radiation with a wide distributionof wavelengths, such as white light. For example, radiation havingwavelengths greater than the plasmon wavelength of the metal islands maybe used. For example, for silver islands, the plasmon wavelength isabout 350 nm. Thus, radiation having wavelengths ranging from about 350nm to microwave wavelengths may be used. If silicon photodetectors areused to detect the radiation, then a preferred incident radiationwavelength range is about 350 nm to about 1100 nm. Preferably, radiationhaving a peak wavelength less than 700 nm, such as 400 nm to 700 nm(i.e., visible light) is used as the incident radiation.

The metal islands 5 may have any suitable thickness such that theislands 5 themselves are opaque to radiation but will generate plasmonenhanced radiation transmission through regions 7. Preferably, metalisland thickness should be at least about two or three times the skindepth of metal. In silver islands with incident radiation in a visiblewavelength range, the skin depth is around 30 nm, and the metal islandthickness should be at least about 60 to 90 nm or greater, preferablygreater than 100 nm. The skin depth increases for longer wavelengthrange and is somewhat different for different metals. Thus, for example,the metal islands 5 may have a thickness of about 50 nm to about 2000nm, such as 100 nm to 400 nm, for example 100 to 250 nm, including from120 to about 180-200 nm. Thus, in the simulation examples shown in FIGS.3A-C, the thickness is preferably 120 nm or greater, such as 120 to 200nm. Thus, the thickness may range from a minimum of 20-30 nm(penetration depth of metal) to a maximum of less than about 10 microns(i.e., the thickness where the absorption of light will be significantin the optical frequency range). Preferably, the thickness is 100 to1,000 nm.

As shown in FIG. 1A, the plurality of metal islands have a width 10,thereby forming an array of transparent regions between the plurality ofmetal islands with a period, a_(o), equal to the width 10 of the islands5 plus the width 9 of the transparent regions. The period a_(o) of thetransparent regions 7 is selected based on the wavelength(s) of theincident radiation that will be used to irradiate the device 1 in orderto enhance the transmission of radiation by plasmon resonance.Preferably, the period of the transparent regions, a_(o), is about threeto ten, such as five to six times as large as the width of thetransparent regions 7. Thus, for a preferred width 9 of about 30 to 100nm of the transparent regions 7 for incident visible light, the perioda_(o) is about less than 1 microns, such as 300 to 900 nm, for example370 nm to about 780 nm. However, the period a_(o) may range from about60 nm to about 2 microns, such as 0.1 to 1.8 microns.

As shown in FIG. 1A, the transparent regions 7 are slit shaped. Theseslits have a length that is significantly larger than the width 9.Preferably, the length is at least ten times larger than the width 9.However, the transparent regions 7 may have any other suitable shapeinstead of a slit shape which results in plasmon enhanced transmissionof radiation.

In an analytical study of the optical transmission through a 2D aperturearray, the initial regime that shows a blue-shift is modeled by theevanescent coupling of the two surface plasmons at the top and bottomsurfaces of a metal layer and the second regime is by decoupled SPs.

In contrast, it is believed that the propagating modes in a slit shapedtransparent region are at least partially responsible for the opticaltransmission through an array of slit shaped transparent regions. Theclear difference between the 1D and 2D array optical transmissioncharacteristics strongly suggests that different mechanisms are involvedin transmitting the light though a slit shaped transparent region thanthrough an aperture in a film.

The transmission spectra in FIG. 2B show three major dips. The minimumtransmission point at around 580 nm tends to stay at nearly the sameposition for different metal island thickness, although the exactlocation cannot be resolved due to an overlap with a neighboring peak.This insensitivity to metal thickness suggests that the phenomenonoccurring at this minimum transmission point involves an interaction oflight primarily with the top or bottom surfaces of metal but not thesidewalls of the metal islands. The SP resonance along the plane thatcomprises the metal/substrate interface of each metal island is expectedto occur at 600 nm wavelength of light, based on a calculation using theformula:

$\begin{matrix}{\lambda = {\frac{L}{m}\sqrt{\frac{ɛ_{d}ɛ_{m}}{ɛ_{d} + ɛ_{m}}}}} & (1)\end{matrix}$

Here, L is the grating period, m is the order of the grating vectorinvolved in SP coupling, and ∈_(m) and ∈_(d) are the dielectricconstants of metal and adjacent dielectric (i.e., a quartz substrate inthis case). This number calculated for m=1 reasonably well matches theminima observed in FIG. 2B. Similarly the transmission minimum at around430 nm well corresponds to the SP resonance at the air/metal interface,which is expected to occur at 430 nm according to the formula above,although an exact position cannot be clearly resolved due to an overlapwith the bulk plasmon wavelength (about 360 nm) at which metal islandsare significantly transparent.

It should also be noted that the sample with 120 nm thick Ag metalislands show a clear, well-defined major dip at around 800 nm, whichcorresponds to significantly longer wavelength than that of thetransmission minima related to the metal/substrate interface.Considering that a slit shaped transparent region structure allowspropagating modes (or vertical SPs along the metal island sidewalls), itis possible that the SP waves on the top and bottom surfaces of a metalisland couple to each other via these island sidewalls. The SPs are thenexpected to resonate along the island surface, i.e., the periphery ofmetal cross-section when the following condition is satisfied along theclose loop:

o∫k _(sp) ·dr=2πm  (2)

Here, m is an integer, and k_(sp) is the SP wave vector and can beexpressed as

$\begin{matrix}{{k_{sp} = {\left( \frac{2\; \pi}{\lambda} \right)\left( \frac{ɛ_{m}ɛ_{d}}{ɛ_{m} + ɛ_{d}} \right)^{1/2}}},} & (3)\end{matrix}$

where λ is the free-space wavelength of incident light.

Along the periphery of metal cross-section, the magnitude of the SP wavevector k_(sp) varies depending on the dielectric material interfacingwith a metal, i.e., either air or quartz in this case. Due to theirregular geometry of the metal cross-section, an approximation may beused to calculate the total phase change along the periphery of themetal islands. If one assumes a simple geometry of circularcross-section with radius r_(o), surrounded by a homogeneous dielectric,then the resonance condition in Equation 2 is reduced to k_(sp)r_(o)=m.For an approximation of r_(o)=110 nm and 30% of the metal peripheryinterfaces with silica and the rest with air, the resonance wavelengthis calculated to be 820 nm for the dipolar resonance case, i.e., m=1.

This number closely matches the location of the transmission dip (800nm) of the sample with 120 nm thick Ag island, as shown in FIG. 2B. Theminimum transmission point shifts to longer wavelength as the metalisland thickness is increased. This behavior is also consistent with theresonance condition discussed above. It is important to note that thissurface plasmon resonance is a phenomenon highly localized at each metalisland and differs from the SP resonance that occurs along the planesthat comprise either the top or bottom surfaces of an array of metalislands.

The angular dependence of both transmission and reflection at a fixedwavelength (633 nm) using a He—Ne laser are shown in FIG. 2C.Transmission and reflection are measured as a function of incidenceangle at 633 nm wavelength (TM polarized) for the 1D array sample with120 nm thick Ag islands. The results shown in FIGS. 2B and 2C suggestthat the three major transmission minima in FIG. 2B can be ascribed tothe SP resonances that involve different sections of the metal surfaces.For a TM polarized light at this wavelength, the transmission spectrashows a minimum when the incidence angle is 45 degrees (the dashed curvein FIG. 2C). This angular position well matches the value (43 degrees)that is calculated from the condition for SP excitation at the planethat comprises the metal/substrate interfaces, i.e., k_(sp)=k_(o) sinθ±mK_(g), where k_(o) is the wave vector of an incident beam, θ is theincidence angle measured from the substrate normal, and K_(g) is thegrating vector. The reflection spectra (solid curve) shows a maximumwith a sharp peak profile (with the full-width-half-maximum of 2-3degrees) at the same incidence angle. The power loss, calculated as thedifference between the incident power and the transmitted plus reflectedpower, is minimal at the SP resonance point. While it is possible thatthis result can be attributed to the diffraction-related Wood's anomaly,which occurs at close proximity to the SP resonance point, it is morelikely that SP resonance plays a dominant role in thistransmission/reflection anomaly.

Thus, without wishing to be bound by a particular theory, the presentinventors believe that surface plasmon resonance is responsible for theobserved transmission minima, involving two different modes ofinteraction with the metal island arrays: 1) the SP resonance along theplanes that comprise either the metal/air or metal/substrate interfaces,and 2) the SP resonance localized along the surface (i.e., the metalisland sidewalls) that encloses each metal island separated by slitshaped transparent regions. At these resonance points, little or nonet-power flows along the metal surfaces and thus there is little or nofunneling of incident power into a slit shaped transparent region. Theincident power then strongly reflects back from the metal surfacewithout incurring any major loss of power. For the case of relativelythin metal islands, the peak (i.e., passband) transmission through thearray is believed to be due to that SP excitation is off-tuned from theresonance points such that a net power flow along the metal surfacesfunnels into a slit region and then decouples into radiation modes whichform a propagating transmitted beam. Thus, the resonance points of theSPs localized at metal islands can be tuned independent of the gratingperiod by selecting metal island thickness and/or transparent regionwidth, and may be used to tailor the transmission characteristics of thearrays, as described below.

The effect of the width 9 of the transparent regions 7 on thetransmission characteristics of the radiation through the metal islandarray will now be described. The present inventors have determined thatfor band pass filters with high selectivity, the transparent regionwidth should vary between about one and three times the penetration orskin depth of SP fields in the metal islands, when the incidentradiation is directed onto the metal islands from an air/metal islandinterface.

The present inventors believe that a peak transmission corresponds to asituation where SP resonance is slightly off-tuned from that at themetal/substrate surface. The surface plasmons localized at each metalisland are then expected to couple each other via a tunneling process asthe transparent region width is reduced. The coupling between thesurface plasmons in neighboring islands is expected to depend on thedegree of overlap of the SP fields across a slit shaped transparentregion, which is determined by the spatial extension of SP fieldscompared to the transparent region width (i.e., gap size).

The skin or penetration depth of SP fields is expressed as follows fromH. Raether, Surface Plasmons (Springer-Verlag, New York, N.Y., 1988)page 6:

${\frac{\lambda}{2\; \pi \sqrt{ɛ_{d}}}\sqrt{\frac{ɛ_{m}^{\prime} + ɛ_{d}}{ɛ_{d}}}\mspace{14mu} {in}\mspace{14mu} {{dielectric}\left( ɛ_{d} \right)}}\;$$\frac{\lambda}{2\; \pi \sqrt{ɛ_{m}^{\prime}}}\sqrt{\frac{ɛ_{m}^{\prime} + ɛ_{d}}{ɛ_{m}^{\prime}}}\mspace{14mu} {in}\mspace{14mu} {{metal}.}$

Here, λ is the free space wavelength of light, ∈_(m)′ is the real partof metal's dielectric constant and Ed is the dielectric constant of themedium adjacent to the metal. The field strength decays by 1/e from thepeak value at the surface. Thus, the penetration depth is dependent onboth the wavelength and the materials through which the radiation istransmitted.

Once strong coupling occurs between metal islands, the metal islandsbecome virtually connected (despite a gap) from the surface plasmonoscillation point of view. Propagation of SPs through the transparentregion will eventually be blocked and the transmission spectra wouldshow profiles similar to those of metal without a transparent region. Itis thus expected that the metal island array would act as a narrow bandpass filter by keeping the main (passband) transmission high whilesuppressing the long-wavelength transmission, when the transparentregion width is within the range of about one to about three times thepenetration depth of the SP fields in the metal islands.

The effect of the metal island thickness on the transmissioncharacteristics of the radiation through the metal island array will nowbe described. The width of the main passband peak decreases withdecreasing metal island thickness.

The main passband peak width is basically determined by the separationof the two transmission dips around the peak, i.e., the resonancewavelength of SP at the metal/dielectric interface and that of the SPlocalized at metal islands. Whereas the former wavelength is determinedmostly by the grating period itself, the latter is governed by othermechanism, basically involving the periphery of cross-section of a metalisland. While keeping the lateral dimension of the metal island close tothe grating period (about a one to three skin depth length shorter thanthe grating period, in order to achieve good bandpass characteristics),the vertical dimension (i.e., thickness) of the metal island may bevaried in order to adjust the total periphery. The change in thicknessof the islands changes the resonance wavelength of the localized SPs andthus the passband width. Use of smaller thickness of metal islands thusreduces the passband width.

Examples of bandpass characteristics of the metal island array of thefirst preferred embodiment are illustrated in FIGS. 2B and 2D. FIG. 2Dillustrates a transmission spectra of silver metal islands whosethickness is 180 nm (i.e., intermediate between those of the 120 and 200nm thick Ag islands whose spectra are shown in FIG. 2B). The minimumtransparent region width of this device is measured to be about 50 nm.In contrast, the minimum widths of the transparent regions of thedevices having 200 nm and 120 nm thick islands are about 30 nm and50-100 nm, respectively. In the devices of examples 1-3, the metalislands are deposited by angled deposition onto a ridged substrate.Thus, increased metal island thickness leads to a decreased transparentregion width. However, for metal island arrays made by other methods,the width of the transparent regions does not necessarily decrease withincreasing thickness of the metal islands.

As shown in FIG. 2D, peak transmission of over 70% (for TM polarization)is observed, even higher than that of the device having 120-nm thickislands, while maintaining the long-wavelength transmission low ataround 20%. Thus, (I_(longer-wavelength)/I_(main))<0.4, preferably lessthan or equal to 0.3, where I_(main) is the intensity of the mainpassband peak and I_(longer wavelength) is the intensity of thetransmitted radiation at longer wavelengths than the main passband peak.As shown in FIG. 2B, very high transmission (nearly the same level asthe peak transmittance of the main passband at around 650 nm) in thelong wavelength range (beyond 950 nm) is visible for the device with 120nm thick islands and about 30 nm wide transparent regions. Thetransmission in the long wavelength regime, however, dramaticallydecreases (from 60% level to 10% transmission for TM polarization) forthe device with 200 nm thick islands and 50-100 nm wide transparentregions. However, the peak (passband) transmission for this device alsosignificantly decreased.

In the case of Ag/air interface at λ=600 nm, the penetration depth iscalculated to be 390 nm in the air or 25 nm in metal. In the case ofAu/air interface at λ=600 nm, the penetration depth is calculated to be280 nm in the air and 30 nm in metal. FIGS. 2B, 2D and 39A-C illustratethat the preferred transparent region width is between 40 and 80 nm,such as about 50 nm, for example, to achieve narrow band passcharacteristics while maintaining high transmission of the main peak. Incontrast, a device with a 30 nm wide transparent regions, showed narrowband pass characteristics, but a lower main peak transmission intensity.A device with a 120 nm wide transparent regions is believed not toexhibit narrow passband characteristics in view of the high intensity ofthe transmission at longer wavelengths, as shown in FIG. 3C.

Thus, the preferred range of minimum transparent region width (around40-80 nm for desirable bandpass characteristics) that is provided inFIGS. 2B and 2D shows a close match to the penetration depth of SPfields in metal (or approximately 1/10 of the air-side penetrationdepth). The preferred range of transparent region width for narrowbandpass filters is found to be approximately one to three penetrationdepths of SP fields in the metal side (or about 1/10 to 3/10 of thepenetration depth in air).

The peak radiation transmission for unpolarized light is estimated to beup about to 50%, which is the theoretical maximum for the unpolarizedlight. This corresponds to near 100% transmission for a TM-polarizedlight. Taking into account the transparent region fill-factor, thetransmission efficiency is greater than 100%, such as 100% to 500%through each transparent region. The upper bound may also be greaterthan 500% because it is determined by the inverse fill factor (i.e.,period/slit width).

A symmetric configuration may be used to reduce a passband width (i.e.,to reduce the number of sidelobes or sidebands) if desired. In thisconfiguration, a second substrate composed of the same dielectric mediaas the first substrate 3 is attached over the top of the metal islands5, such that the metal islands 5 have interfaces with the samedielectric media on both sides, as illustrated in FIG. 4.

As discussed above, the metal islands 5 may comprise discrete metalislands that are not connected to each other (i.e., the metal islandsare not in direct contact with each other) or metal islands that areconnected to each other at a peripheral region of the optical device.For example, as shown in FIG. 5, the metal islands 5 are discrete metalislands. In contrast, as shown in FIG. 6, the metal islands 5 areconnected to each other at the periphery of the devices 1.

Preferably, the metal islands 5 are formed by self assembly and arelocated on a plurality of ridges 21 on the transparent substrate 3.Preferably, as shown in FIG. 7, each one of the plurality of metalislands 5 is located on a corresponding one of the plurality of ridges21. The metal islands and the ridges may have any suitable shape, asdiscussed above. Preferably, the metal islands and the ridges are shapedsuch that the openings 7 between the islands are slit shaped. Thus, alength of each metal island is preferably at least 10 times larger thanits width and a length of each ridge is preferably at least 10 timeslarger than its width. As shown in FIG. 7, the plurality of ridges 21preferably have a rectangular shape. The ridges 21 may compriseprotrusions on the upper portion of the radiation transparent substrate3 protrusions on the upper portion of a radiation transparent layerlocated on the radiation transparent substrate, or discrete radiationtransparent elements located over the radiation transparent substrate orlayer. Thus, the substrate 3 may comprise a unitary substrate (i.e., asingle layer radiation transparent material) or it may contain more thanone layer of radiation transparent material.

Preferably, each respective metal island 5 extends over an upper surface23 of each ridge 21 and over at least a portion of at least one sidesurface 25 of each respective ridge 21. Most preferably, the metalislands are formed by angled deposition, as shown in FIG. 8. In thiscase, each metal island 5 extends lower over a first side surface 25 ofa respective ridge 21 than over a second side surface 27 of therespective ridge 21 because the metal is angle deposited from the firstside surface 25, as will be described in more detail below.

In an alternative aspect of the present invention, the substrate 3comprises a nanopore array. Preferably, the substrate 3 comprises ananodic aluminum oxide nanopore array located over a radiationtransparent substrate material, as will be described in more detailbelow.

The optical devices 1 of the preferred aspects of the present inventionmay be made by any suitable method where a plurality of metal islands 5are formed on the radiation transparent substrate 3. As described above,the metal islands 5 are preferably selectively deposited on theplurality of ridges 21, such that metal is not deposited between theridges 21.

FIG. 8 illustrates a preferred method of selectively forming the metalislands 5 by self assembly using angled deposition. In this method, themetal is directed onto the ridges 21 in a non perpendicular directionwith respect to tops of the ridges. For example, if the ridges contain aflat upper surface 23, then the metal may be directed at an angle of 20to 70 degrees, such as 30 to 50 degrees, with respect to the flat uppersurfaces 23 of the ridges.

Preferably, the metal islands 5 are deposited on the ridges 21 byevaporation (thermal or electron beam), as shown in FIG. 8. In theevaporation method, the metal is evaporated thermally or by an electronbeam from a metal source or target 31 onto the substrate 3. For angleddeposition, the substrate 3 is inclined by 20 to 70 degrees, such as 30to 50 degrees, preferably 45 degrees, with respect to the target 31.Since the spaces between the ridges 21 are sufficiently small, no metalis deposited between the ridges during the angled deposition. Thus, thetilt angle theta of the substrate should be sufficient to prevent metaldeposition between the ridges 21. The metal islands 5 may also bedeposited by any other suitable angled or nonangled metal depositionmethod, such as metal organic chemical vapor deposition (MOCVD),molecular beam epitaxy (MBE), sputtering and other suitable methods.

The ridges 21 may be formed on the substrate 3 by any suitable method.Preferably, the ridges are made using lithography. Most preferably, theridges are made using photolithography, as will be described in moredetail below. However, the ridges 21 may be made by using imprint ornanoindentation lithography such as, by stamping a transparent unitaryor multilayer substrate with a ridged stamp to form a plurality ofridges and grooves in the transparent substrate.

FIGS. 9A, 9B and 9C illustrate one preferred method of forming theridges in a transparent substrate (i.e., a unitary substrate or amultilayer substrate) 3 using photolithography. As shown in FIG. 9A, aphotoresist layer 41 is formed on the first surface of the substrate 3.The term “photoresist layer” includes any suitable positive or negativephotosensitive layer used for semiconductor and other microdevicepatterning. The photoresist layer 41 is then selectively exposed byradiation, such as UV or visible light, or by an electron beam.

The selective exposure can take place through a mask, by selectivelyscanning a narrow radiation or electron beam across the photoresistlayer 41 or holographically. For example, as shown in FIGS. 9B and 9C,the photoresist layer may be separately exposed holographically for eachcell of the wavelength separation device or the entire layer may beexposed at the same time for a chirped grating pattern.

To perform holographic lithography, a laser beam is split into twobeams. The two beams are then reflected so that they converge togetheronto the photoresist layer 41. Where the two beams converge, aninterference pattern comprised of multiple parallel lines of intenselight is generated. The parallel lines of intense light occur with aparticular periodicity which may be adjusted by changing the incidentbeam angle. Further adjustment of the periodicity may be accomplished bychanges in optics, e.g., changes in the wavelength of the light source,and/or the refractive index of the ambient dielectric adjacent to thephotoresist. Thus, the photoresist is exposed where the two beamsconverge and not exposed where the two beams do not converge. Thelength, A, shown in FIG. 9B is equal to the peak wavelength of the splitlaser beams divided by (sin θ₁+sin θ₂), where θ₁ and θ₂ are the anglesof the laser beams with the normal to the photoresist surface, as shownin FIG. 9A.

The selective exposure leaves the photoresist layer 41 with exposed andnon-exposed regions. The holographic exposure is preferred because itforms slit shaped exposed and non-exposed regions in the photoresistlayer 41 which can then be used to form slit shaped ridges and groovesin the substrate.

The exposed photoresist layer 41 is then patterned, as shown in FIG. 9B.If the photoresist layer 41 is a positive photoresist layer, then theexposed regions are removed by a suitable solvent, while leaving theunexposed regions as a photoresist pattern 43 on the substrate 3, asshown in FIG. 9B. If the photoresist layer 41 is a negative photoresistlayer, then the unexposed regions are removed by a suitable solvent,while leaving the exposed regions as a photoresist pattern 43 on thesubstrate 3.

The upper surface of the substrate 3 is then etched to form the ridgesusing the patterned photoresist layer 41 as a mask (i.e., using theexposed or non-exposed regions 43 remaining on the substrate as a mask).The substrate may be patterned by wet and/or dry etching. It should benoted that other intermediate processing steps, such as photoresistbaking, cleaning, etc., may also be added as desired.

Furthermore, if desired, a hardmask layer, such as a silicon nitride,silicon oxide, silicon oxynitride or a metal layer, such as a chromiumlayer, may be added between the photoresist layer 41 and the substrate 3if needed, as shown in FIGS. 9D-9I. As shown in FIGS. 9D and 9E,hardmask layer 42, such as a Cr layer, is formed on the substrate 3. Aphotoresist pattern 43 is then formed on the hardmask layer 42 by anysuitable method, such as the holographic lithography method, as shown inFIG. 9F. The hardmask layer 42 is then patterned using the photoresistpattern 43 as a mask to form a hardmask pattern 44, and then thephotoresist pattern 43 is removed, as shown in FIG. 9G. The substrate 3is then patterned to form the ridges 21 using the hardmask pattern 44 asa mask, as shown in FIG. 9H. The hardmask pattern 44 is then removed.The metal islands 5 are then selectively deposited on the ridges 21,such as by angled deposition, as shown in FIG. 9I.

A preferred example of the parameters of the method described above isas follows. An about 40 nm thick Cr hardmask layer is deposited on aquartz substrate by thermal evaporation. This is followed by HMDSapplication and photoresist spin coating to a thickness of about 100 nmon the hardmask layer. Microposit Photoresist 1805 and Microposit Type PThinner in 1:1 volume ratio is used with a spin speed 5000 rpm. Thephotoresist layer was then subjected to a softbake at 95 degrees Celsiusfor 30 minutes. The photoresist is exposed by holographic lithography. AUV He—Cd laser (325 nm wavelength, 15 mW CW power) is used for theexposure. The photoresist layer is then developed using Microposit 351and DI water in 1:4 volume ratio. The developed (i.e., patterned)photoresist is then subjected to a hardbake at 120 degree Celsius for 30minutes.

The Cr hardmask layer is then etched using the patterned photoresistlayer as a mask. The Cr layer is etched using a reactive ion etching(RIE) system (PlasmaTherm 790 ICP/RIE) in a two step etching process. Instep 1, Cl₂ (20 sccm)+O₂ (10 sccm) at 10 mTorr pressure, RIE power of 25W and ICP power of 100 W for 30 seconds are used. In step 2, Cl₂ (24sccm)+O₂ (6 sccm) at 10 mTorr pressure, RIE power of 10 W and ICP powerof 100 W for 7 minutes are used.

The patterned hardmask layer is then used as a mask to pattern thequartz substrate. The quartz substrate is etched by RIE using CF₄ (37sccm)+O₂ (4 sccm) at 15 mTorr, RIE power of 100 W and ICP power of 150 Wfor 12 minutes. Thereafter, the remaining Cr hardmask is removed bychemical etching with NaOH+K₃Fe(CN)₆+H₂O solution. The Ag islands arethen deposited on the mesa etched substrates using angled deposition.The Ag islands are deposited to various thicknesses using thermalevaporation of Ag source in a base pressure of 10⁻⁵ Torr with a tiltangle of 45 degrees. The holographically-patterned and mesa-etchedsubstrates, once made, can be utilized as a master mold innanoimprinting the array patterns on substrates without involving anyseparate optical or electron lithography process each time of patterndefinition or transfer.

FIGS. 10A and 10B illustrate another preferred method of forming theridges in a transparent substrate (i.e., a unitary substrate or amultilayer substrate) 3 using photolithography and a nanopore array. Oneexemplary method of forming a nanopore array is described in Z. Sun andH. K. Kim, Appl. Phys. Lett., 81 (18) (2002) 3458.

First, as shown in FIG. 10A, a photoresist pattern 43 in a shape of agrating is formed on the substrate 3 in the same manner as describedabove and as illustrated in FIGS. 9A-9B. The photoresist pattern may beformed by holographic or non-holographic lithography. After forming thephotoresist pattern 43, the substrate 3 may be etched to transfer thegrating pattern to the substrate to form ridges 21 illustrated in FIG.7, after which the photoresist pattern 43 is removed. Alternatively, thesubstrate etching and photoresist pattern removal steps may be omitted.

A metal layer 51 capable of being anodically oxidized is conformallydeposited over the ridges 21, if the ridges are present, or over thephotoresist pattern 43, if the photoresist pattern has not been removed,as shown in FIG. 10B. The conformally deposited metal layer 51 assumesthe grating pattern of the underlying substrate or photoresist pattern,as shown in FIG. 10B. In other words, the metal layer 51 is formed on agrating patterned transparent substrate (i.e., a ridged substrate or apatterned photoresist 43 covered substrate) such that the gratingpattern of the substrate 3 is translated to an upper surface of thefirst metal layer 51.

The metal layer 51 may comprise any suitable metal, such as Al, Ta, Ti,Nb and their alloys, which may be anodically anodized. The metal layer51 may be deposited by any suitable method, such as sputtering, MOCVD,evaporation (thermal or electron beam), MBE, etc. The metal layer 51 mayhave any suitable thickness, such as 100 to 1000 nm, preferably 350-400nm. The corrugation depth in the upper surface of the metal layer 51 ispreferably about the same as the corrugation depth of the substrate orthe photoresist pattern. Preferably, the corrugation depth of the metallayer 51 is about 20 to about 300 nm, such as about 80 to 100 nm.

The metal layer 51 then is oxidized anodically, by any suitable method.For example, an Al layer 51 on a silica substrate 3 may be anodicallyoxidized in dilute electrolyte (1H₃PO₄+800H₂O in volume ratio) at roomtemperature using a platinum wire as a counter electrode. Theanodization is preferably conducted under a constant voltage mode forabout 40 minutes. The anodic voltage is chosen such that the expectedpore distance matches the grating period, for example 140 volts for a350 nanometer grating period. In a naturally-formed alumina pore array,the interpore distance is proportional to the anodization voltage, i.e.about 2.5 nanometers/volt. After anodization, the samples are preferablytreated with phosphoric acid (diluted with water in a 1:3 volume ratio)for one to two minutes. FIG. 10C is a electron micrograph of a nanoporearray 53 grown in the grating patterned aluminum layer 51 when thealuminum layer 51 is converted to aluminum oxide by anodic oxidation.The resulting alumina pores exhibit a uniform depth, such as about 100to 2000 nm, preferably about 300 to 400 nm and the pore bottom has aconcave, hemispherical shape with barrier thickness of about 100 to 300nm, such as 150 to 200 nm. The preferred pore diameter is about 5 to 100nm, such as 5 to 10 nm. The nanopores selectively form in troughs of thegrating pattern in the upper surface of the anodically oxidized metallayer 51.

After forming the nanopore array 53, such as the array shown in FIG.10C, metal islands 5 are selectively grown in the nanopores, as shown inFIG. 10D. One preferred method of selectively growing metal islandsinside the nanopores in a metal oxide layer is an electroplating methodillustrated in FIG. 10E. The nanopore array 53 is formed on a conductiveor a semiconducting substrate 63. The substrate 63 may comprise a metallayer, such as a metal layer which is not anodically oxidized, or adoped semiconductor layer, such as silicon, gallium arsenide or galliumnitride. The substrate 63 may comprise the radiation transparentsubstrate 3 used in the devices 1 or the substrate 63 may comprise atemporary substrate which is transparent or non-transparent toradiation. The substrate 63 and array 53 are then provided into anelectroplating bath 65 containing a liquid metal 67. A potentialdifference (i.e., a voltage) is applied between the substrate 63 and thearray 53. Since the array 53 is thinner in regions 55 below thenanopores 57, a voltage gradient exists in these regions 55. This causesthe metal 67 from bath 65 to selectively deposit into the nanopores 57.If desired, the electroplating method may be used to selectively fillthe nanopores 57 with metal 67 from bath 65. The metal 67 may be anymetal which exhibits the previously described plasmon enhancement effectand which may be deposited into metal oxide pores by electrodeposition,such as Ni, Au, Pt and their alloys. Thus, the islands 5 are formed byfilling the nanopores 57 with the electroplated metal 67.

In an alternative preferred aspect of the present invention, thenanopores 57 are filled only part of the way with the metal 67 duringthe electroplating step. In this case, the metal 67 may be any metalwhich can act as a catalyst for selective metal vapor deposition. Forexample, the metal 67 may be Au. The array 53 with the catalyst metal 67formed on the bottom of the nanopores 57 is then transferred to a metalvapor deposition chamber, such as a chemical vapor deposition chamber.Metal islands 5 are then selectively grown on the catalyst metal 67 byselective vapor deposition. The metal islands 5 may comprise any metalwhich exhibits the previously described plasmon enhancement effect andwhich may be selectively deposited on a catalyst metal 67, but not onmetal oxide walls of the nanopore array 53. For example, this metal maycomprise Al or Ag.

If the nanopore array 53 is formed on a temporary substrate 63, then thetemporary substrate may be removed from the array 63 before or after theformation of the metal islands 5 on the array 53. The temporarysubstrate may be removed by selective etching, polishing or chemicalmechanical polishing of the substrate, by selective etching of a releaselayer (not shown for clarity) located between the temporary substrate 63and the array 53, or by peeling the substrate 63 away from the array 53.In case of peeling, one or more peel apart layers may be located betweenthe substrate 63 and the array 53. The peel apart layer(s) have a lowadhesion and/or strength such that they can be separated mechanicallyfrom each other or from the array and/or the substrate. The transparentsubstrate 3 is then attached to the array 53 before or after forming themetal islands 5 on the array, on the same and/or opposite side of thearray 53 from where the temporary substrate 63 was located.

In an alternative, preferred aspect of the present invention, the metalislands 5, are formed by angled deposition on the ridges of a nanoporearray, as described above and as shown in FIG. 8. In another alternativeaspect of the present invention, a metal layer is deposited over thenanopore array such that metal extends into the pores, and the metallayer is then chemically mechanically polished or etched back to exposetop portions of the nanopore array. The polishing or etch back stepleaves discrete metal islands in the nanopores, separated by the metaloxide nanopore array transparent regions.

In another alternative aspect of the present invention, the nanoporearray is formed without first patterning the substrate 3 or forming thephotoresist pattern 43. In this aspect, a metal layer 51, such as an Al,Ta, Ti or Nb layer is deposited on the unpatterned substrate. Thencorrugations are formed in the metal layer 51 by any suitable method.For example, the corrugations may be formed by selective laser ablationof the metal layer, by nanoindentation or nanoimprinting, or byphotolithography (i.e., by forming a photoresist pattern on the metallayer, then etching the metal layer using the pattern as a mask andremoving the photoresist pattern). Preferably, holographicphotolithography is used to pattern the metal layer 51, and a temporarysilicon nitride, silicon oxide or silicon oxynitride hard mask is usedbetween the photoresist and the metal layer 51. Then, the metal layer 51is anodically oxidized as described above.

FIGS. 11A-D illustrate an alternative method of forming the metalislands using a templated nanopore array. As shown in FIG. 11A, themetal oxide nanopore array 53 on substrate 63 is formed using the methoddescribed above and illustrated in FIGS. 10A-10C. Then, a conformaltemplate material 71 is deposited over the array 63, as shown in FIG.11B. The conformal template material 71 may comprise any material whichcan conformally fill the nanopores 57 of the array 53. For example, theconformal template material 71 may comprise silicon oxide, siliconnitride, a glass heated above its glass transition temperature, a CVDphospho- or a borophosphosilicate glass (PSG or BPSG, respectively), aspin on glass or a polymer material. If desired, the conformal templatematerial may comprise all or part of the transparent substrate 3.

Then, as shown in FIG. 11C, the conformal template material 71 isremoved from the nanopore array 53. The conformal template material 71contains ridges 73 which previously extended into the nanopores 57 ofthe array. Then, the metal islands 5 are selectively deposited into thepores 75 between the ridges 73 of the conformal template material 71using the electroplating method or on the ridges 73 using angleddeposition method as described above. If the conformal template material71 is the transparent substrate 3 material, then the process stops atthis point. If the conformal template material 71 is not the transparentsubstrate 3, then the conformal template material 71 is separated fromthe metal islands 5 by any suitable method, such as selective etching,polishing or chemical mechanical polishing. The metal islands 5 areattached to the transparent substrate 3 before or after removingmaterial 71.

FIGS. 12A and 12B illustrate an alternative method of forming the metalislands 5 without using ridges on a substrate and without using ananopore array. In this method, a metal layer 81 is formed on thesubstrate 3, as shown in FIG. 12A. The substrate 3 may contain featureson its upper surface or it may contain a flat upper surface. The metallayer 81 is then patterned into a plurality of metal islands 5 as shownin FIG. 12B. The metal layer 81 may be patterned lithographically asdescribed previously. Thus, a photoresist layer 41 is formed on a firstsurface of the metal layer 81. The photoresist layer is selectivelyexposed to form exposed and non-exposed regions. The exposed photoresistlayer is patterned into pattern 43 and the metal layer is etched intothe plurality of islands 5 using the patterned photoresist layer as amask.

The photoresist layer may be exposed holographically ornon-holographically. If desired, an optional, temporary hardmask layerdescribed above may be formed between the metal layer 81 and thephotoresist. Alternatively, the metal layer may be patterned byselective laser ablation or other non-photolithographic methods insteadof by photolithography.

FIGS. 13A, 13B and 13C illustrate an alternative lift off method offorming the metal islands 5. This method also does not require usingridges on a substrate or a nanopore array. In this method, a photoresistlayer 41 is formed on the substrate 3 as shown in FIG. 13A. Thesubstrate 3, may contain features on its upper surface or it may containa flat upper surface. The photoresist layer is selectively exposed toform exposed and non-exposed regions. The photoresist layer may beexposed holographically or non-holographically.

The exposed photoresist layer 41 is then patterned to form a photoresistpattern 43, exposing portion of the upper surface of the substrate 3. Asshown in FIG. 13B, a metal layer 81 is formed over the photoresistpattern 43 and over exposed portions of the upper surface of thesubstrate 3.

As shown in FIG. 13C, the photoresist pattern 43 is then lifted off,such as by selective etching or other suitable lift off techniques.Portions of the metal layer 81 located on the photoresist pattern 43 arelifted off with the pattern 43 to leave a plurality of metal islands 5on the upper surface of the substrate 3.

In order to improve further the passband characteristics of the opticaldevices 1, a three dimensional stacked structure of metal island layers91, 93 may be used, as shown in FIG. 14. Two pieces of single-layer 1Doptical devices 91, 93 are vertically stacked face-to-face, with gratinglines (i.e., slit shaped transparent regions) 97 substantially parallelto each other, and with the spacing between faces in a far field regime,where the far field regime comprises spacing that is greater than about3 to 5 times the wavelength of the incident light or radiation. Thetransparent regions 97 may be slightly offset from each other by anamount that still allows radiation transmission through both layers. Thetwo metal island layers are then expected to interact in the far-fieldregime, and therefore the overall transmission would be simply a productof two transmission profiles. This will result in suppression of lowerintensity side peaks and also will narrow the width of the main peak.

The transmission spectra through the transparent regions in the threedimensional Ag metal island array is shown in FIG. 15. The vertical axiscorresponds to the transmission ratio, P_(out)/P_(in). The measurementresult shown in FIG. 15 confirms that the side peaks are wellsuppressed. The main peak is also narrowed from 170 nm to 140 nm in FWHMvalue. The three dimensional structures are not limited to just twolayers and may have any suitable number of layers greater than two andvarious different layer patterns and interlayer spacing.

If desired, the optical devices 1 may include an integrated radiationsource, such as a laser, LED or lamp adapted to emit the incidentradiation and/or an integrated a radiation detector, such as a chargecoupled device (CCD) array or CMOS active pixel array, adapted to detectthe radiation transmitted through the substrate and between theplurality of metal islands. Alternatively, a separate radiation sourceand/or the radiation detector may be used with the devices 1.

The optical devices 1 may be used for any suitable application. Thus,the devices 1 may be used as a nano-optic filter with a narrow passbandwidth or as a polarizer. The devices 1 may also be used for wavelengthseparation of incident radiation. Specifically, by using differenttransparent region periods or different surface feature spacing on themetal islands (such as chirped periods or spacing), the devices 1 may beused for wavelength separation applications described below. Forinstance, if the period of the transparent regions is different indifferent cells or regions of the devices, then the devices may be usedas an ultra compact monochromator or in a spectrometer, in a chip scalemultispectral imaging device (i.e., color camera) or in fluorescencesensing system.

The devices 1 may be used for other applications, such as a lightcollector, collimator or coupler for an optical fiber, a light selectiondevice in near field optical scanning microscope, and a photolithographymask.

For example, the present inventors have realized that optical devices,such as compact wavelength separation devices including monochromatorsand spectrum analyzers, as well as multispectral imaging systems andoptical analyte detection systems may be based on plasmon resonanceenhancement of radiation effect. The period of openings in metal islandsare varied in different portions or cells of the metal film or islandsto form a two dimensional wavelength separation device portion of theimaging and detection systems.

The wavelength separation device includes a plurality of metal islands,having a two dimensional array of a plurality of slit shaped openingshaving a width that is less than a wavelength of incident radiation tobe provided onto the islands. The islands are configured such that theincident radiation is resonant with at least one plasmon mode on themetal islands. The enhanced radiation transmitted through the openingshas at least two passband ranges with two different peak wavelengths,and preferably three or more, such as ten or more, different passbandranges with different peak wavelengths.

FIG. 19 is schematic illustration of wavelength separation using astacked one dimensional (1D) slit array as a micron-scale monochromatordevice 101 of the second embodiment. FIG. 20A illustrates the top of thedevice 101. As shown in FIG. 19, incident radiation having a range ofwavelengths λ₁ to λ_(n) is provided onto a metal island array 105 havinga plurality of openings 107. The openings have a width that is less thanat least one wavelength of incident radiation, such that the incidentradiation is resonant with at least one plasmon mode on the metalislands. The transmitted radiation is provided through the plurality ofopenings such that the transmitted radiation is simultaneously separatedinto a plurality of passbands having different peak wavelengths λ_(i),λ_(j), and λ_(k). The incident radiation may be provided onto eitherside of the array 105.

Preferably, radiation having a peak wavelength less than 700 nm, such as400 nm to 700 nm (i.e., visible light) is used as the incidentradiation. In this case, the openings 7 have a width of 100 nm or less,such as 30 to 100 nm. In the case of incident radiation with longerwavelengths, such as infrared radiation, the openings may have aproportionally larger width.

In this device 101, a metal layer or film 105 is formed over a radiationtransparent substrate 103 and then patterned into a plurality of islandswhich may be connected in the peripheral region. However, a freestanding metal membrane film without a supporting substrate orunconnected metal islands on a substrate may be used instead. Forexample, FIG. 20B illustrates a wavelength separation device 1containing unconnected metal islands 5 separated by transparent regions7.

The metal islands 5 in array 105 are separated by slit shaped openings107 that are periodically arranged with a cellular pattern. The slitspreferably have a length that is at least ten times larger than thewidth.

The metal island array 105 is divided into a desired number of cells orregions 108, such as at least two cells, where the grating period of theopenings 107 is substantially the same within each cell. However, thegrating period of the openings 107 differs between cells. In otherwords, the openings 107 in each cell are spaced apart from adjacentopenings in the cell by about the same distance. However, this distanceis different for different cells. For example, three cells 108A, 108Band 108C are illustrated in FIG. 19.

The grating period of the openings 107 in each cell 108 is designed toproduce a passband at a certain peak wavelength in the transmissionspectrum. Thus, a transmission of the radiation having one peakwavelength is enhanced due to the period of the openings in the firstcell 108A. A transmission of the radiation having a different peakwavelength is enhanced due to the different period of the openings inthe second cell 108B.

Preferably, the device 101 contains at least ten cells, more preferablyat least 30 cells, such as 30 to 3,000 cells, for example 30 to 1,000cells. A period of openings in each of the cells is different thanperiods of openings in each of the other cells. The transmission ofpassband radiation having a different peak wavelength through openingsin each cell is enhanced due to the period of the openings in therespective cell. Preferably, the passband radiation transmitted througheach cell 108 has a peak wavelength that differs by at least 1 nm, suchas by at least 10 nm, for example by 10 to 100 nm, from peak wavelengthsof radiation transmitted through the other cells 108.

The propagation length of surface plasmons is estimated to be about 5 toabout 10 microns. A cell size comparable to this number or larger ispreferred because it allows sufficient plasmon interaction. A 10-μmcell, for example, corresponds to about 30 periods of gratings when0.5-μm peak passband wavelength is assumed. The cell size may be greaterthan 5 microns, such as greater than or equal to 10 microns, for example10 to 10,000 microns, and the number of gratings per cell varies by cellsize and peak passband wavelength.

A cell 108 size of about 10 microns, such as 5-20 microns is preferredbecause it matches a typical pixel size of commercially available CCDdevices. For high array density (i.e., for better spatial resolution),it is desirable to keep the cell size as small as possible. However, forease of fabrication, the cell size may be increased to about 50 to 500μm. The overall size of a N-channel monochromator array 101 will then beapproximately N×(50-500) μm. The N-channel monochromator arraypreferably has N cells 108, where N is an integer between 10 and 10,000.

Preferably, a period of openings in each cell ranges from about 250 nmto about 700 nm and a width of each opening preferably ranges from about20 nm to about 80 nm for visible light incident radiation. The width ofthe openings 107 may be larger for infrared incident radiation.

An alternative design to the 1×N array pattern described above is toutilize a chirped grating (i.e., opening) pattern. In other words, thegrating period (i.e., the period of the openings) is continuouslychirped over a distance, L. If a radiation detector is used with thewavelength separation device, then the detector pixel size, W, definesthe effective cell size of a wavelength separation device, such as afilter, and the total number of channels of the array will be L/W. Anadvantage of this design is that the entire monochromator array can beimplemented with a single holographic lithography process, as will bedescribed below.

The wavelength separation devices, such as the monochromators ornano-optic filter arrays can be made ultra compact, and the wavelengthseparation can be achieved in an ultra-compact space. For example, thedimensions can be made as small as a micrometer-scale area along the adirection transverse to the radiation propagation direction (i.e.,length) and virtually zero length (i.e., the thickness of stackedlayers, such as less than 0.1 microns) along the longitudinal (i.e.,radiation propagation) direction, without being restricted by thediffractive optics. Preferably, the monochromator length, width andthickness are each less than 1 cm. More preferably, the monochromatorlength is less than 100 microns and its thickness is less than 10microns.

In the case of slit shaped openings between metal islands, (i.e., a 1Dgrating case), the optical transmission through the sub-wavelengthopenings depends on the polarization of incident light. For the TEpolarized light (i.e., where the E-field is parallel to the gratinglines), for example, surface plasmons are not excited due to theunavailability of grating vectors along the E-field direction, since thesurface plasmons are longitudinal waves. Therefore, transmission for TEpolarizations is expected to be much lower than TM polarization. Thispolarization dependence can be utilized for detecting the polarization(and its spatial distribution) of incident light. Alternatively, thewavelength separation device can be used as a polarizing filter.

Any suitable metal such as Ag, Al, Au and Cu may be used to form themetal islands. Preferably, metals, including Ag, Al, Au, Cu or theiralloys, which exhibit a bulk plasmon frequency in the 9-10 eV range areused. This makes the plasmon-induced phenomena observable in a broadspectral range (Vis-to-IR). Al and Cu are common metals used asinterconnect metallization in integrated circuit chips andphotodetectors. Thus, the metal islands of the wavelength separationdevice may be made using the same semiconductor manufacturing equipmentas used to form chips and photodetectors.

In a third preferred embodiment of the present invention, the wavelengthseparation devices 301 of the first two embodiments are used togetherwith a photodetector 302 to form a spectrum analyzer device 304, asshown in FIG. 21. Any device which can detect visible, UV and/or IRpassband transmitted radiation may be used as the photodetector 302. Thephotodetector 302 is adapted to detect radiation transmitted through thewavelength separation device 301.

Preferably, an array solid state photodetector cells, such as asemiconductor photodetector array is used as a photodetector. Mostpreferably, charge coupled devices (CCDs), a CMOS active pixel sensorarray or a focal plane array are used as the photodetector. Thephotodetector 302 shown in FIG. 21 includes a substrate 313, such as asemiconductor or other suitable substrate, and a plurality ofphotosensing pixels or cells 306. Preferably, each photodetector cell orpixel 306 is configured to detect passband radiation having a given peakwavelength from each respective cell of the wavelength separation device301. The wavelength separation device 301 includes the metal islands 305and an optional radiation transparent substrate 303.

The photodetector 302 can be optically coupled (i.e., in contact or inproximity) to the output plane of the metal islands 305 for detection ofthe near-field output through the metal islands. The output of eachdetector cell is then electronically addressed for display andprocessing. A processor, such as a computer or a special purposemicroprocessor, is preferably provided to determine an intensity ofradiation detected by each cell of the photodetector. Thus, thephotodetector 302 is preferably optically coupled to the metal islands305 without utilizing diffractive optics between the wavelengthseparation device and the photodetector.

In a preferred aspect of the invention, the spectrum analyzer 304thickness in a radiation transmission direction is less than 1 cm andthe spectrum analyzer 304 length perpendicular to the radiationtransmission direction is less than 1 cm.

In a fourth preferred embodiment of the present invention, thenanophotonic monochromator/spectrum analyzer can be used as amultispectral imaging system, when the monochromator is extended to atwo dimensional array configurations. A multispectral imaging system isa system which can form an image made up of multiple colors. One exampleof a multispectral imaging system is a digital color camera which cancapture moving and/or still color digital images of objects orsurroundings. Another example of a multispectral imaging system is aninfrared camera, which forms a digital image in visible colors ofobjects emitting infrared radiation, such as a night vision camera. Thecamera contains a processor, such as a computer, a special purposemicroprocessor or a logic circuit which forms a color image (i.e., asdata which can be converted to visually observable image or as an actualvisually observable image) based on radiation detected by thephotodetector. The multispectral imaging system may store the colorimage in digital form (i.e., as data on a computer readable medium, suchas a computer memory or CD/DVD ROM), in digital display form (i.e., as astill or moving picture on a screen) and/or as a printout on a visuallyobservable tangible medium, such as a color photograph on paper.

FIGS. 22A, 22B and 22C show examples of a multispectral imaging system404 comprising a three dimensional wavelength separation device 401 anda photodetector 402. The wavelength separation device 401 comprises a 2Dmosaic arrangement of metal islands 405 that allows multi imaging withspatially resolved polarization detection capability. The system 404contains an array of cells 408 arranged in two dimensions in thewavelength separation device 401. Preferably, the cells 408 are arrangedin a rectangular or square matrix layout. However, any other layout maybe used instead. Each cell 408 is adapted to produce a multicolorportion of a multidimensional image.

Each cell 408 comprises at least three subcells 418. Each subcell 418 ina particular cell 408 is designed to transmit one particular color (or anarrow IR, VIS or UV radiation band). Preferably, each subcell 418comprises metal islands 405 with slit shaped openings 407 having a givenfirst period. This first period of the openings is different from theperiods of at least some of the other subcells 418 in a given cell 408.In this case, each subcell 418 in a particular cell 408 is designed totransmit one particular color (or a narrow IR or UV radiation band) witha certain polarization. In other words, each subcell 418 allowsradiation having a given narrow band of wavelength to pass through. Forexample, the narrow band of wavelengths may correspond to a particularcolor of visible light radiation. Each cell 418 of the 2D array 404 ispreferably identical to the other cells in the array because each cellcontains the same arrangement of subcells 418.

For example, FIG. 22B illustrates a system 404A containing thirty twocells 408 (8×4 array of cells 408A, 408B, 408C, etc.) in the wavelengthseparation device. Each cell 408 contains six subcells 418. Each subcellis designed to transmit one particular color with a certain polarizationto detector 402. Three subcells 418A, 418B, 418C have slit shapedopenings 407 oriented in a first direction (such as a horizontaldirection). Another three subcells 418D, 418E and 418F have slit shapedopenings oriented in a second direction perpendicular to the firstdirection (such as a vertical direction). Thus, each cell 408 with thissubcell layout can transmit both TM and TE polarized light. In onepreferred aspect of this embodiment, the period of the openings 407 ineach pair of subcells (418A and 418D, 418B and 418E, 418C and 418F) isthe same. The subcells in each pair of subcells have slit shapedopenings oriented perpendicular to each other to detect TE and TMpolarizations of each color. However, the period of openings 407 isdifferent between each pair of subcells. Thus, the system 404A shown inFIG. 22B is a three color imaging system, where each pair of subcells isadapted to transmit one color.

FIG. 22C illustrates a system 404B containing nine cells 408 (3×3 arrayof cells) in the wavelength separation device. Each cell 408 containstwelve subcells 418. In one preferred aspect of this embodiment, theperiod of the opening in each pair of subcells is the same. The subcellsin each pair of subcells have slit shaped openings orientedperpendicular to each other to detect TE and TM polarizations of eachcolor. However, the period of openings is different between each pair ofsubcells. Thus, the system 404B shown in FIG. 22C is a six color imagingsystem, where each pair of subcells is adapted to image one color.

The subcells 418 are arranged in each cell 408 in a square orrectangular matrix. However, any other suitable arrangement may be used.The three- or six-color-separation systems were described above forillustration purposes only. By increasing the number of subcells in eachcell, the system can be easily scaled up for high-resolutionmultichannel analyzers with more than six color separation and imaging.Furthermore, the cells 408 may be located in contact with adjacentcells, as shown in FIG. 22B or separately from adjacent cells, as shownin FIG. 22C. While the mosaic arrangement of subcells 418 within asingle cell 408 allows for multicolor separation capability, repeatingthe mosaic cell into a 2D array results in an array of spectrumanalyzers on a single chip to form the multispectral imaging system,such as a color camera.

Furthermore, the multispectral imaging systems also contain aphotodetector 402, as described above. Preferably, the photodetector 402contains one pixel or cell 406 for each cell 408 in the metal islands.Most preferably, each photodetector pixel 406 is arranged on a substrate413 in registry with each cell 408, such that each photodetector pixel406 receives radiation transmitted through only one cell 408.

In a fifth preferred embodiment of the present invention, thenanophotonic spectrum analyzer or the multispectral imaging systemdescribed above are used in an optical analyte detection system. Ananalyte detection system is a system in which radiation from the analyteis detected by the spectrum analyzer or the multispectral imagingsystem. The analyte may be an organic material, such as a biomaterial(i.e., protein, antibody, antigen, etc.) or a polymeric material, or aninorganic material, such as a metal, glass, ceramic or semiconductormaterial. The analyte may be in any one or more of solid, liquid or gasstates. Any suitable radiation from the analyte may be detected by theanalyte detection system, such as fluorescence or luminescence from theanalyte, absorption or transmittance of incident radiation which passesthrough the analyte or is reflected from the analyte, or modification ofthe incident radiation by the analyte, such as peak shifting in theradiation that is transmitted through or reflected by the analyte.

FIG. 23A illustrates an exemplary optical analyte detection system 500.The system 500 includes an excitation source 501, an analyte holder 502and either the one dimensional spectrum analyzer 304 or the twodimensional multispectral imaging system 404 described above.

Any suitable excitation source 501 may be used. Preferably, an opticalexcitation source, such as a light emitting diode, laser or lampemitting in the UV, visible or IR range is used. However, any othernon-optical excitation source may be used instead, which generates anoptical response 504, such as fluorescence, from the analyte 503. Forexample, the excitation source may comprise a thermal source, such as aheater or furnace, which causes the analyte to emit radiation inresponse to heat. Alternatively, an X-ray, gamma ray or electron beamsource may be used as an excitation source if the X-rays, gamma rays orelectrons cause the analyte to emit radiation.

The analyte holder 502 may comprise any device that can hold the analyte503 during the optical detection. For example, as shown in FIG. 22A, theanalyte holder 502 may comprise a microslide if the analyte 503 is aliquid, solid or gel biomaterial, such as a serum sample. Alternatively,the analyte holder 502 may comprise a radiation transparent gas orliquid container for a gas or liquid analyte, or any suitable shelf,susceptor or support for a solid or gel analyte 503.

The system 500 includes a one dimensional spectrum analyzer 304 (i.e.,the monochromator and photodetector combination described above) todetect the radiation 504 from the analyte 503 if two dimensionalresolution of the analyte is not required. The system 500 includes thetwo dimensional multispectral imaging system 404 described above if itis desirable to detect radiation from a two dimensional distribution ofanalyte 503. For example, the multispectral imaging system may detectdifferences in radiation 504 emitted by different regions of the analyte503 and/or may be used to detect radiation 504 from a larger portion ofthe analyte 503.

FIG. 22B illustrates another embodiment of the optical analyte detectionsystem 500. In this system 500, the excitation source 501 comprises alight emitting diode or a laser diode. An optional nano-optic excitationfilter 506 is placed between the excitation source 501 and the analyteholder 502. The filter 506 may comprise metal islands containingsubwavelength slit shaped openings 507 having the same period, in orderto polarize the incident radiation. Alternatively, the filter maycomprise another type of polarizing filter that polarizes the incidentradiation. The analyte holder 502 comprises a microslide and the analyte503 comprises a biomaterial, such as a protein, antibody and/orfluorophore containing sample. The multispectral imaging system 404includes the photodetector 402 and the wavelength separation device 401described above. Preferably, the openings 407 in the wavelengthseparation device 401 are slit shaped and are oriented in a directionperpendicular to the openings in the filter 506. Thus, the openings 407prevent the polarized excitation source 501 radiation from reaching thephotodetector 402, and the photodetector detects fluorescence from theanalyte 503 which passes through the openings 407. In other words, thegrating lines of the two dimensional nano-optic monochromator 401 arealigned perpendicular to those of the excitation filter 506 so that theunabsorbed incident (i.e., excitation radiation) is significantlyfiltered out before reaching the detector array 402.

The nano-optic filters show an acceptance angle of about +5-10 degrees.A spacing between about 200 to about 2000 microns between the nano-opticmonochromator array 401 and the analyte holder 402 is expected to bereasonable for an about 10 to about 100 micron cell size of thewavelength separation device (i.e., nano-optic array) 401. This spacingprovides sufficient space to slide a plate shaped analyte holder 502with the analyte 503 in and out of the system 500.

As discussed above, the nano-optic monochromator array 401 can beintegrated with a detector chip 402 in a hybrid or monolithic fashion asdiscussed above. In the hybrid configuration case, commerciallyavailable detector chips (CCDs or CMOS active pixel sensor arrays) maybe used. The number of cells 408 (i.e., channels) in the nano-opticmonochromator 401 may be kept relatively small, such as 10 to 100 cells.However, a larger number of cells, such as 100 to 10,000 cells may beused. The size of each cell may be about 50 to about 500 microns, whichis 5 to 50 times larger than the pixel size of commercial photodetectorarrays (typically about 10 microns in CCDs). A particular wavelengthcomponent of a fluorescence signal that passes through a cell 408 isthen detected by about 5 to about 50 pixels. Alternatively, each cell408 may be designed to correspond to one photodetector pixel. In thecase of monolithic integration of the monochromator 401 andphotodetector 402, the nano-optic monochromator may be located in aportion of the metal interconnect of a CMOS active pixel array (or of aCCD) chip. Therefore the entire process is compatible with the standardCMOS process.

FIG. 24 illustrates a method of using the optical analyte detectionsystem 500 for medical analysis according to a preferred embodiment ofthe present invention. However, the system 500 may be used on otheranalytes and/or for other purposes.

As shown in FIG. 24, a bio analyte 503, such as blood or other human oranimal body fluid, is provided onto the analyte holder 502. The analyte503 contains various components of interest, such as proteins,antibodies, etc. The analyte holder 502 contains an array of one or moretypes of attachment members 508, such as various antibodies, antigen,proteins etc. For example, the attachment members 508 may comprisespecific antibodies to various disease proteins, such as influenza,smallpox and anthrax proteins. Alternatively, the attachment members 508may comprise specific antigen or protein to various disease antibodies.

In one preferred aspect of this embodiment, these antibodies arefluorescently labeled with any suitable fluorophore, such as an organicdye molecule or a semiconductor quantum dot. When the analyte 503contains antigen or proteins that specifically bind to the antibodies508, these antigen or proteins bind to the antibodies 508. The bindingchanges the characteristic of the radiation 504 emitted by thefluorophore in response to the excitation radiation. For example, thewavelength and/or intensity of the radiation 504 emitted by thefluorophore may be changed by the binding. The photodetector 402 detectsthe radiation 504 and a computer or other processor 509 stores,transmits and/or displays the results of the detection by thephotodetector. For example, when the radiation 504 from the fluorophoreattached to the anthrax specific antibody 508 changes, the computer 509indicates that the analyte blood 503 came from a patient who is infectedwith anthrax.

The binding may be detected by one or more of the following methods. Inthe first method, different attachment members are provided ontodifferent regions of the analyte holder 502, and this layout informationis provided into the computer 509. The analyte holder 502 containingattachment members 508 is irradiated with exciting or incident radiation501 and the fluorescence radiation 504 of the fluorophores is detectedby the photodetector 402 as the background radiation. Then, the analyte503 is provided onto the analyte holder 502 and the analyte holder isagain irradiated with the exciting radiation 501. The photodetectordetects the fluorescence radiation 504 and the computer 509 determinesif the fluorescence radiation 504 changed from any region on the analyteholder from before to after the placement of the analyte 503. Thecomputer can thus determine if there was binding to the specificattachment members 508 in a particular region of the analyte holder 502,and thus determine the content of the analyte 503 since the attachmentmembers 508 are different in different regions of the analyte holder502. If desired, the exciting radiation 501 may be directed onto theanalyte holder 502 continuously to detect real time binding between theproteins or antigen in the analyte 503 and the attachment members 508.

In another method to detect the binding, a fluorophore having adifferent fluorescence wavelength is attached to each type of attachmentmember 508 and this data is stored in the computer 509. The sampleholder 502 containing attachment members 508 is irradiated with excitingor incident radiation 501 and the fluorescence radiation 504 of thefluorophores is detected by the photodetector 402 as the backgroundradiation. Then, the analyte 503 is provided onto the analyte holder 502and the analyte holder is again irradiated with the exciting radiation501. The photodetector detects the fluorescence radiation 504 and thecomputer 509 determines if the fluorescence radiation 504 of aparticular wavelength changed from before to after the placement of theanalyte 503. The computer can thus determine if there was binding to thespecific attachment members 508 based on the wavelength of thefluorophore radiation that was changed after the introduction of theanalyte 503. If desired, the exciting radiation 501 may be directed ontothe analyte holder 502 continuously to detect real time binding betweenthe proteins or antigen in the analyte 503 and the attachment members508. In this method, it is preferred, but not necessary to locatedifferent types of fluorophores/attachment members 508 on differentregions of the analyte holder 502, since the wavelength rather thanlocation of the changed radiation is used to detect binding. It shouldbe noted that the intensity of the detected radiation may be used todetermine the degree of binding between the analyte contents and theattachment members 508, if desired.

A third detection method is illustrated in FIG. 24. In this method, thefluorophores are not attached to the attachment members 508. Instead,additional fluorescently labeled members 510, such as fluorescentlylabeled antibodies, antigen or proteins, are provided onto the analyteholder 502 after the analyte 503. These members 510 are designed to bindto proteins, antigen or antibodies found in the analyte 503. Thus, ifthe antibodies, antigen or proteins form the analyte 503 are bound tothe attachment members 508, then the fluorescently labeled members 510also bind to these antibodies, antigen or proteins form the analyte 503.The presence of the bound antibodies, antigen or proteins form theanalyte 503 is determined by irradiating the analyte holder 502 withexciting or incident radiation 501 and the fluorescence radiation 504from the members 510 is detected by the photodetector 402. The differenttypes of fluorescently labeled members 510 may be labeled withfluorophores which emit radiation of a different wavelength and/ordifferent types of attachment members 508 may be located in differentparts of the analyte holder 502 in order to distinguish a type ofprotein, antibody or antigen that bound to the attachment members 508.

In this method, while it is preferable to include the attachment members508, these members 508 may be omitted. Instead, the surface of theanalyte holder 502 may be treated to attach all proteins, antibodies,antigens or other analyte components of interest, and the differenttypes of fluorescently labeled members 510 labeled with fluorophoreswhich emit radiation of a different wavelength are provided onto theanalyte 503. Members 510 are designed to only bind to specificcomponents of the analyte. If these analyte components are not present,then members 510 will not remain on the analyte holder 502. Thus,presence of a particular component of the analyte may be detectedwithout attachment members 508 by determining the wavelength(s) ofradiation emitted by the attached labeled members 510.

The overall system 500 performance is expected to be determined by thefollowing factors: the power and spectral characteristics of both theexcitation source and fluorophores, the detector 402 responsivity,spacing between component layers and the filter characteristics of boththe excitation filter 506 and monochromator array 401. In the case ofmultispectral fluorescence using organic dye fluorophores, each dyeusually requires different excitation wavelength. LEDs can be used withnano-optic excitation filters shown in FIG. 19 in order to produce awavelength-multiplexed beam that has a well-defined narrow spectralwidth at each component wavelength. In the case of quantum dotfluorophores, the fluorophores of different wavelengths can be excitedwith a single exciting wavelength. This simplifies excitation opticscompared with using organic dye fluorophores.

An advantage of the system 500 is its capability to simultaneouslydetect multiwavelength components of fluorescence signals utilizing thefine resolution of the nano-optic monochromator 401 in conjunction withquantum dot or nanotube probes of narrow spectral width. Thismultispectral detection allows an application of a deconvolutiontechnique in extracting each wavelength component from mixed-wavelengthsignals. This is further refines the spectral analysis capabilities ofthe system 500.

Another advantage of the system 500 is high throughout. For example, asshown in FIGS. 22B and 22C, by using two dimensional monochromators 401having a 4×8 or 3×3 array configuration, respectively, allowssimultaneous analysis of 8×4 or 3×3 analyte arrays. The 2D arrays, whoseindividual cells possessing multispectral analysis capability, offer anultimate high-throughput.

The one and two dimensional spectrum analyzers 304, 404 may be made byany suitable method. For example, the wavelength separation device 301,401 and the photodetector 302, 402 may be manufactured separately andthen bonded or attached together to form the analyzer. For example, thewavelength separation device 301, 401 and the photodetector 302, 402 maybe attached to each other by a radiation transparent layer or adhesiveand/or by a fastening device, such as a bracket. The wavelengthseparation device 301, 401 and the photodetector 302, 402 may beattached to each other at the periphery or along their entire length.The wavelength separation device 301, 401 may contact the photodetector302, 402 directly, or a radiation transparent layer, such as a siliconoxide or glass layer, or the substrate 303 may be placed between them.

In another preferred aspect of the third embodiment, the spectrumanalyzer device is formed monolithically. In other words, rather thanforming the wavelength separation device 301, 401 and the photodetector302, 402 separately and then attaching them to each other, theindividual components or layers of one of the wavelength separationdevice 301, 401 and the photodetector 302, 402 are formed sequentiallyover the other. Thus, the individual components or layers the wavelengthseparation device 301, 401 may be formed sequentially over thephotodetector 302, 402 and vise versa.

For example, the solid state photodetector array 302, 402 is provided inor over a substrate 313. This step preferably includesphotolithographically forming a CCD, a CMOS active pixel array or afocal plane array in or on the substrate 313. In other words, thephotodetector array 302, 402 may be formed by standard microfabricationtechniques, such as semiconductor, metal and/or insulating layerdeposition, ion implantation, photoresist masking, and etching of theunmasked layer portions.

A metal film 305, 405 is then monolithically deposited on thephotodetector array 302, 402 (i.e., the metal film is deposited by athin film deposition method, such as evaporation, sputtering or CVDrather than being formed and then attached to the array 302, 402). Themetal film 305, 405 is then photolithographically patterned to form aplurality of openings therein to form the metal islands. The openingsmay be formed by forming a photoresist layer on the metal film or over ahardmask layer over the metal film, exposing and patterning thephotoresist layer, and then etching the uncovered portions of the metalfilm to form the openings.

Alternatively, the plurality of metal islands are monolithicallydeposited onto the photodetector array 302, 402. A number of suitableisland deposition methods may be used, as discussed above.

If the metal islands contain a periodic or quasi-periodic surfacetopography, then the topography may be photolithographically formed onthe metal islands.

In a preferred aspect of the invention, the wavelength separation deviceis formed at the same time as the metallization of the photodetector.For example, the metal islands 305, 405 may be formed over a interlayerinsulating layer which is formed over metallization or interconnects ofthe photodetector 302, 402. In CCD, CMOS or focal plane arrayphotodetectors, one or more levels of metallization interconnects areformed over the semiconductor devices. The wavelength separation device301, 401 may be formed over the metallization layers, between themetallization layers, as part of one of the metallization layers (i.e.,a portion of a metal level acts as the wavelength separation device andanother portion acts an interconnect for the photodetector), below themetallization layers, or on the opposite side of the substrate 313 fromthe metallization layers.

For example, the wavelength separation device may comprise an Al film orislands and may comprise a portion of the Al interconnect parts of astandard CMOS process. In the 0.13-μm CMOS process, for example, five orsix levels of metal interconnects are used. These interconnects can bedesigned as the nano-optic monochromator arrays and be monolithicallyintegrated with CMOS active pixel arrays on the same chip. Thenano-optic filter arrays can be designed to cover the spectral range ofapproximately 400 to 1000 nm, by using grating periods of 250 to 700 nm.Thus, the spectrum analyzer chips 304, 404 can be fabricated using asemiconductor foundry service.

In the devices of the preferred embodiments, a symmetric configurationmay be used to reduce a passband width (i.e., to reduce the number ofsidelobes or sidebands) if desired. In this configuration, thewavelength separation device is sandwiched between two radiationtransparent substrates composed of the same dielectric media.

Specific Examples

The following specific examples illustrate preferred embodiments of thepresent invention and should not be considered limiting on the scope ofthe invention.

The devices of examples 1, 2 and 3 are made by the same process, exceptthat the metal island thickness is 200 nm in example 1, 120 nm inexample 2 and 180 nm in example 3. Since the metal islands are depositedby angled deposition, the width of the transparent regions increasedwith decreasing metal island thickness. Thus, the minimum width of thetransparent regions in example 1 is about 50 to 100 nm (the width variesdue to the slight non-uniformity of the metal islands), the minimumwidth of the transparent regions in example 2 is about 30 nm and theminimum width of the transparent regions in example 3 is about 50 nm.The transmission spectra of the devices of examples 1 and 2 with 120 and200 nm thick metal islands, respectively, are shown in FIG. 2A and thetransmission spectra of the device of example 3 with 180 nm islands isshown in FIG. 2C. FIGS. 16A, 16B and 2A illustrate SEM micrographs ofmetal island arrays with metal island thicknesses similar to those ofexamples 1-3. The thickness of metal islands in FIG. 16A is 400 nm, inFIG. 16B, 250 nm and in FIG. 2A 180 nm. The devices of examples 1, 2 and3 are made by the method illustrated in FIGS. 9D-9I.

FIG. 17 shows a micrograph of a one dimensional (1D) silver metal islandarray device of Example 4 with narrow slit shaped transparent regions.The device is formed by depositing a 200-nm-thick Ag on a1D-grating-etched quartz substrate. The grating pattern is generatedwith a holographic process and the grating period is designed to be 750nm. The slit width is measured to be around 150 nm at the narrowestpart. This corresponds to about a 20% ratio of the transparentregion/metal island surface area for normal incident waves.

FIG. 18 illustrates transmission spectra through the transparent regionsin the Ag metal island array. The vertical axis corresponds to thetransmission ratio, P_(out)/P_(in) for unpolarized light. For TMpolarization, the peak transmission is over 90%. The dependence of thetransmission spectra on the incidence angle of incident radiation isalso shown in FIG. 18. As the incidence angle is varied, thetransmission peaks shift and split. The main passband peak shows afull-width-half-maximum value of about 170 nm. A much narrower passbandwidth of about 10 nm to about 160 nm and well suppressed transmission atlong wavelength is possible with a different transparent region designand improved uniformity, and with an optimized metal island thicknessand slit width, using a numerical analysis of transmission spectra basedon a transfer matrix and a quasi-analytical model.

FIG. 25 illustrates a top view of the experimental set up for examples5-12. As shown in FIG. 25, a single or double layer wavelengthseparation device 301 comprising metal islands containing a plurality ofopenings is positioned over a line camera 302 containing a plurality ofpixels 306. The incident or input light beam 315 area is larger thanthat of the wavelength separation device 301, such that some of theincident light 315 is detected by pixels 306 of the camera 302 withoutpassing through the wavelength separation device.

FIG. 26 illustrates the transmission spectra for examples 5, 6 and 7.FIG. 26 is a plot of the ratio of the transmitted to incident radiationversus wavelength of the radiation. In example 5, white light wastransmitted through a metal island array having about 2100 Angstromthick silver islands. The openings between the islands have a gratingperiod (dg) of about 401 nm. The grating period is also referred toherein as the period of the transparent regions, a_(o), as shown by peak(a) on the left of FIG. 26, the transmitted radiation through this arrayhas a peak wavelength of about 676.2 nm. In example 6, white light wastransmitted through a metal island array having about 2100 Angstromthick silver islands. The openings between the islands have a gratingperiod (dg) of about 478 nm. As shown by peak (b) in the middle of FIG.26, the transmitted radiation through this array has a peak wavelengthof about 789.6 nm. In example 7, white light was transmitted through ametal island array having about 2100 Angstrom thick silver islands. Theopenings between the islands have a grating period (dg) of about 552 nm.As shown by peak (c) on the right of FIG. 26, the transmitted radiationthrough this array has a peak wavelength of about 912.8 nm. Thus, asillustrated in FIG. 26, radiation of a different peak wavelength istransmitted through the array depending on the grating period of theopenings in the array.

FIG. 27 illustrates a transmission spectra for white light passedthrough prior art 450 nm, 650 nm and 880 nm filters. FIG. 28 illustratesthe results of example 8. In example 8, the wavelength separation deviceof example 5 was placed next to the wavelength separation device ofexample 7. In other words, the metal island array having grating period(dg) of about 401 nm is placed at a first arbitrary location on thecamera 302 (i.e., at a location between 8000 and 10,000 microns from areference point), while the metal island array having grating period(dg) of about 552 nm is placed at a second arbitrary location on thecamera 302 (i.e., at a location between 10,000 and 12,000 microns from areference point). White light is then passed through the 450 nm priorart filter illustrated in FIG. 27 and then through the arrays ofexamples 5 and 7. The transmitted light is then detected by the camera302. As shown in FIG. 28, the large peaks at about 7,000 and about13,000 microns corresponds to the light which was not transmittedthrough the arrays. Furthermore, as shown in FIG. 28, the arrays ofexamples 5 and 7 were effective in filtering 450 nm peak wavelengthlight. This is to be expected because the peak transmission of thearrays of examples 5 and 7 is 676.2 nm and 912.8 nm, respectively.

FIG. 29 illustrates the results of example 9. The conditions of example9 are identical to those of example 8, except that white light waspassed through the 650 nm prior art filter of FIG. 27 rather than the450 nm prior art filter. As shown in FIG. 29, the array example 7 waseffective in filtering 650 nm peak wavelength light because the peaktransmission of this array is 912.8 nm. In contrast, the array ofexample 5 transmitted a portion of the 650 nm light because the peaktransmission of this array is 676.2 nm.

FIG. 30 illustrates the results of example 10. The conditions of example10 are identical to those of example 8, except that white light waspassed through the 880 nm prior art filter of FIG. 27 rather than the450 nm prior art filter. As shown in FIG. 30, the array example 5 waseffective in filtering 880 nm peak wavelength light because the peaktransmission of this array is 676.2 nm. In contrast, the array ofexample 7 transmitted a portion of the 880 nm light because the peaktransmission of this array is 912.8 nm.

As discussed above, the wavelength separation devices 101, 301 may havea constant grating period (dg) in each cell or a chirped grating periodalong the length of the device. FIG. 31A illustrates an example of awavelength separation device where each cell contains a differentgrating period d1, d2, d3 and d4. FIG. 31B is a micrograph of such adevice. FIG. 32A illustrates an example of a wavelength separationdevice where the grating period is chirped. FIG. 32B illustrates anexemplary plot of grating period versus location (x) on the wavelengthseparation device.

FIG. 33A schematically illustrates example 11 having a two layerwavelength separation device. The device contains an array with achirped grating period 91 which is stacked over an array with a constantgrating period 93. Is should be noted that array 93 may be stacked overarray 91 instead if desired. Any suitable grating periods may beselected. Preferably, the chirped grating period of array 91 overlaps aconstant grating period d1 of array 93 in at least one location, x1, ofthe wavelength separation device. This is shown in FIG. 33B, which is aplot of grating period versus location on the device, x. One of thedetector pixels 306 is positioned at location x1 in the camera 302. Thetotal transmittance, T, of white light through both arrays 91 and 93(shown in FIG. 34C) is a product of transmittance T1 through the firstarray 91 (shown in FIG. 34B) and the transmittance T2 through the secondarray 93 (shown in FIG. 34A). As can be seen from FIGS. 34A-C, theradiation transmitted through both arrays (i.e., T) has a narrower peakwidth than the radiation transmitted through each of the arrays 91, 93alone (i.e., T1 or T2). Thus, stacking two arrays reduces a peak widthof the transmitted radiation as well as reduces the intensity of thesidebands or side peaks relative to the intensity of the main peak.

FIG. 35A schematically illustrates example 12 wherein two arrays 91, 93having a chirped grating period are stacked over each other. Thedetector pixels 306 are located along different locations on the camera302. Thus, each pixel 306 window (a 7 micron window for example)captures radiation of a different peak wavelength that passed through adifferent portion of the wavelength separation device that has adifferent grating period of the chirped grating period. For example,FIG. 35A illustrates the case where the arrays 91 and 93 have the samechirped grating period. The arrays 91, 93 may be stacked such that thetransparent regions in each array are aligned with the transparentregions in the adjacent array or the arrays 91, 93 may be stacked suchthat transparent regions in one array are offset by a predeterminedamount from the transparent regions in the other array in the horizontal(i.e., x) direction along the direction of the pixels 306. FIG. 35Billustrates transmittance spectra for various arrangements of chirpedarrays. The peak labeled “No offset” corresponds to transmittance ofwhite light through both arrays 91, 93 at a predetermined location overthe detector (x=290 microns) where there is no offset betweentransparent regions of the chirped arrays 91, 93. The grating period ofthe arrays is 340 nm at this location (x=290 microns). The peak labeled“40 micron offset” corresponds to transmittance of white light throughboth arrays 91, 93 at a predetermined location over the detector (x=330microns) where there is a 40 micron offset between transparent regionsof the chirped arrays 91, 93. The grating period is 400 nm at thislocation. As can be seen from these two peaks, by introducing the offsetbetween the arrays, the peak width is narrowed, but the overall peakintensity is decreased.

FIG. 36A illustrates a transmission spectra of a single-layerwavelength-separation device of example 13 having a chirped gratingperiod along the length of the device (see FIG. 32A). The device is 800micron wide (along the length direction) and comprises nine cells orfilters. Each filter is 98 microns wide and has a different (butconstant) grating period, linearly chirped from 390 nm to 630 nm with 30nm step between neighboring cells. The transmission spectra measured ateach cell show a progressive and linear shift of the main passbandposition from around 750 nm to around 1100 nm. The peaks listed in thelegend of FIG. 36A are shown in the Figure from right to left (i.e., thedg=630 nm “black” peak is the right most peak and the dg=390 nm “violet”peak is the left most peak).

FIG. 36B illustrates a transmission spectra of a two layer stackedwavelength separation device of example 14 comprising two pieces of thesame device shown in FIG. 36A. Enhancement of bandpass characteristicsof the stacked configuration is clearly observed. FIG. 36 illustrates asuppression of lower intensity side peaks at the shorter wavelengthregion, reduced transmission in the longer wavelength region, andreduction of the main passband width from about 150-200 nm to about100-150 nm compared to FIG. 36A. Further refinement of bandpasscharacteristics, especially suppression of long wavelength transmissionis expected by optimum control of metal thickness and thus the metalslit width. In the example shown in FIG. 36A, the mesa-etched quartzsubstrate is designed to have a slit width of 120 nm. After angleddeposition of a 150 nm thick Ag layer, the slit width is reduced to50-80 nm. Control of metal thickness can adjust the slit width.

FIG. 36C illustrates wavelength separation with a two layer stackeddevice of example 15, as measured with a linear array CCD detector (seeFIG. 25 for experimental configuration). The wavelength separationdevice is a 390 micron wide array comprising 28 cells (filters) witheach cell having a 14 micron width. The grating period is chirped from360 nm to 630 nm with a 10 nm step along the array direction. A 980 nmwavelength light is incident to a wavelength separation device (twolayer stacked). The light is registered at pixels at around 10850 micronlocation of a CCD array. When a 700 nm light is incident to the samedevice, the light is registered at 10600 micron location of the CCDarray. This spatial separation on CCD matches the spectral separation ofthe two input lights. In other words, the different wavelength of lightis detected by a different portion of the CCD array which is locatedunder the portion of the wavelength separation device designed totransmit light of that particular wavelength.

Various embodiments and preferred aspects of the present invention havebeen separately described above. However, each step or feature from onepreferred embodiment or aspect may be used in another embodiment oraspect, in any appropriate way. All publications, patents and patentapplications mentioned herein are incorporated herein by reference intheir entirety.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andmodifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention. Thedrawings and description were chosen in order to explain the principlesof the invention and its practical application. It is intended that thescope of the invention be defined by the claims appended hereto, andtheir equivalents.

1. A band pass filter comprising a metal nanowire array, wherein: thearray comprises a plurality of nanowires separated by slit shapedradiation transparent regions; the nanowires comprise metal islandslocated over a radiation transparent substrate; and the metal islandshave a thickness of between 100 nm and 250 nm.
 2. (canceled) 3.(canceled)
 4. The filter of claim 1, wherein the metal islands areeither connected to each other in a peripheral region of the filter orare unconnected to each other.
 5. The filter of claim 1, wherein thefilter is adapted to act as a band pass filter due to surface plasmonresonance between incident radiation and the metal islands.
 6. Thefilter of claim 5, wherein the radiation transparent regions have awidth of 100 nm or less.
 7. The filter of claim 6, wherein the radiationtransparent regions have a width of 30 nm to 100 nm.
 8. The filter ofclaim 6, wherein the radiation transparent regions have a length that isat least 10 times larger than their width.
 9. (canceled)
 10. The filterof claim 6, wherein the metal islands have a width of less than 1micron.
 11. The filter of claim 6, wherein the radiation transparentregions have a grating period of between 60 nm and 2 μm.
 12. Aspectrometer comprising the filter of claim
 1. 13. A monochromatorcomprising the filter of claim
 1. 14. A multispectral imaging devicecomprising the filter of claim
 1. 15. An analyte detection systemcomprising the filter of claim
 1. 16. A method of band pass filteringcomprising providing incident radiation comprising a first range ofwavelengths onto the band pass filter of claim 1 such that radiationtransmitted through the filter contains a pass band of wavelengthsnarrower than the first range of wavelengths.
 17. The method of claim16, wherein the incident radiation is resonant with at least one plasmonmode on the metal islands, thereby enhancing transmission of radiationhaving at least one peak wavelength in the pass band of wavelengthsbetween the plurality of metal islands.
 18. The method of claim 16,wherein the incident radiation comprises visible radiation.
 19. Themethod of claim 16, wherein the filter at least partially filters out aportion of the first range wavelengths outside the pass band ofwavelengths by at least 50% more than the wavelengths within the passband of wavelengths.
 20. A band pass filter comprising a metal nanowirearray, wherein: the array comprises a plurality of nanowires separatedby slit shaped radiation transparent regions; the nanowires comprisemetal islands located over a radiation transparent substrate; and themetal islands are either connected to each other only in a peripheralregion of the filter or are unconnected to each other.
 21. The filter ofclaim 20, wherein the radiation transparent regions have a length thatis at least 10 times larger than their width.
 22. The filter of claim20, wherein: the filter is adapted to act as a band pass filter due tosurface plasmon resonance between incident radiation and the metalislands; the radiation transparent regions have a width of 100 nm orless; the metal islands have a thickness of between 100 nm and 250 nm;the metal islands have a width of less than 1940 nm; and the radiationtransparent regions have a grating period of between 60 nm and 2 μm. 23.The filter of claim 20, wherein all nanowires in the metal nanowirearray extend in the same direction.
 24. The filter of claim 20, whereinthe metal islands are connected to each other only in a peripheralregion of the filter.
 25. The filter of claim 20, wherein the metalislands are unconnected to each other.
 26. A spectrometer comprising thefilter of claim
 20. 27. A monochromator comprising the filter of claim20.
 28. A multispectral imaging device comprising the filter of claim20.
 29. An analyte detection system comprising the filter of claim 20.30. A method of band pass filtering comprising providing incidentradiation comprising a first range of wavelengths onto the band passfilter of claim 20 such that radiation transmitted through the filtercontains a pass band of wavelengths narrower than the first range ofwavelengths.
 31. The method of claim 30, wherein: the incident radiationis resonant with at least one plasmon mode on the metal islands, therebyenhancing transmission of radiation having at least one peak wavelengthin the pass band of wavelengths between the plurality of metal islands;the incident radiation comprises visible radiation; and the filter atleast partially filters out a portion of the first range wavelengthsoutside the pass band of wavelengths by at least 50% more than thewavelengths within the pass band of wavelengths.
 32. A band pass filtercomprising a metal nanowire array, wherein the array comprises aplurality of nanowires separated by slit shaped radiation transparentregions; the nanowires comprise metal islands located over a radiationtransparent substrate; and the metal islands have a thickness of lessthan about 2000 nm.
 33. The filter of claim 32, wherein the metalislands are either connected to each other in a peripheral region of thefilter or are unconnected to each other.
 34. The filter of claim 32,wherein: the filter is adapted to act as a band pass filter due tosurface plasmon resonance between incident radiation and the metalislands, wherein the radiation transparent regions have a width of 100nm or less; the metal islands have a thickness of between about 100 nmand about 400 nm; the metal islands have a width of less than 1940 nm;and the radiation transparent regions have a grating period of between60 nm and 2 μm.
 35. The filter of claim 32, wherein the metal islandshave a thickness of between about 20 and 2000 nm.
 36. A spectrometercomprising the filter of claim
 32. 37. A monochromator comprising thefilter of claim
 32. 38. A multispectral imaging device comprising thefilter of claim
 32. 39. An analyte detection system comprising thefilter of claim
 32. 40. A method of band pass filtering comprisingproviding incident radiation comprising a first range of wavelengthsonto the band pass filter of claim 32 such that radiation transmittedthrough the filter contains a pass band of wavelengths narrower than thefirst range of wavelengths.
 41. The method of claim 40, wherein: theincident radiation is resonant with at least one plasmon mode on themetal islands, thereby enhancing transmission of radiation having atleast one peak wavelength in the pass band of wavelengths between theplurality of metal islands; the incident radiation comprises visibleradiation; and the filter at least partially filters out a portion ofthe first range wavelengths outside the pass band of wavelengths by atleast 50% more than the wavelengths within the pass band of wavelengths.